Dissertations / Theses on the topic 'Quantum chemistry theory'
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Kryvohuz, Maksym. "Quantum-classical correspondence in response theory." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/43759.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.Includes bibliographical references (p. 113-118).
In this thesis, theoretical analysis of correspondence between classical and quantum dynamics is studied in the context of response theory. Thesis discusses the mathematical origin of time-divergence of classical response functions and explains the failure of classical dynamic perturbation theory. The method of phase space quantization and the method of semiclassical corrections are introduced to converge semiclassical expansion of quantum response function. The analysis of classical limit of quantum response functions in the Weyl-Wigner representation reveals the source of time-divergence of classical response functions and shows the non-commutativity of the limits of long time and small Planck constant. The classical response function is obtained as the leading term of the h-expansion of the Weyl-Wigner phase space representation and increases without bound at long times as a result of ignoring divergent higher order contributions. Systematical inclusion of higher order contributions improves the accuracy of the h expansion at finite times. The time interval for the quantum-classical correspondence is estimated for quasiperiodic dynamics and is shown to be inversely proportional to anharmonicity. The effects of dissipation on the correspondence between classical and quantum response functions are studied. The quantum-classical correspondence is shown to improve if coupling to the environment is introduced. In the last part of thesis the effect of quantum chaos on photon echo-signal of two-electronic state molecular systems is studied. The temporal photon echo signal is shown to reveal key information about the nuclear dynamics in the excited electronic state surface.
(cont.) The suppression of echo signals is demonstrated as a signature of level statistics that corresponds to the classically chaotic nuclear motion in the excited electronic state.
by Maksym Kryvohuz.
Ph.D.
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Babbush, Ryan Joseph. "Towards Viable Quantum Computation for Chemistry." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:17467325.
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Since its introduction one decade ago, the quantum algorithm for chemistry has been among the most anticipated applications of quantum computers. However, as the age of industrial quantum technology dawns, so has the realization that even “polynomial” resource overheads are often prohibitive. There remains a large gap between the capabilities of existing hardware and the resources required to quantum compute classically intractable problems in chemistry. The primary contribution of this dissertation is to take meaningful steps towards reducing the costs of three approaches to quantum computing chemistry. First, we discuss how chemistry problems can be embedded in Hamiltonians suitable for commercially manufactured quantum annealing machines. We introduce schemes for more efficiently compiling problems to annealing Hamiltonians and apply the techniques to problems in protein folding, gene expression, and cheminformatics. Second, we introduce the first adiabatic quantum algorithm for fermionic simulation. Towards this end, we develop tools which embed arbitrary universal Hamiltonians in constrained hardware at a reduced cost. Finally, we turn our attention to the digital quantum algorithm for chemistry. By exploiting the locality of physical interactions, we quadratically reduce the number of terms which must be simulated. By analyzing the scaling of time discretization errors in terms of chemical properties, we obtain significantly tighter bounds on the minimum number of time steps which must be simulated. Also included in this dissertation is a protocol for preparing configuration interaction states that is asymptotically superior to all prior results and the details of the most accurate experimental quantum simulation of chemistry ever performed.Chemical Physics
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Brooks, A. N. "The quantum theory of atom-triatom reactions." Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316696.
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Gador, Niklas. "Curve-crossing quantum wavepacket dynamics - Experiment and theory." Doctoral thesis, KTH, Physics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3754.
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In this thesis, I present experimental and theoretical workon quantum wavepacket dynamics in potential curve-crossings,using gas-phase Rb2 as working media. Particularly, we havefocused on curve-crossing cases with intermediate strengthcoupling, which leads to complicated wavepacket motion withe.g. large splittings and interference. Previous experiments onsuch systems are scarce.
Experimentally, femto-second pump-probe spectroscopy wasperformed using two independent optical parametric amplifiers.A near-effusive Rb2molecular beam source was developed to produce astable, high density and collision-free beam. Pump-probefluorescence was detected using an optical assembly designedfor good collection efficiency.
Theoretically, analysis of experimental data was aided byquantum dynamical calculations. The used numerical simulationprogram is powerful in its ability to include any number ofstates with coupling elements, together with a fully timepropagated pump pulse-molecule interaction. It was furtherdeveloped to include molecular rotation as a centrifugalcorrection term to the potential curves, and to do statisticalthermal averaging to permit direct comparison withexperiment.
Our work on the Rb2A-state system is a pioneering femto-secondexperimental curve-crossing study of a system of twointermediately coupled bound electronic states. The wavepacketfragments, following different roads, meet and interfere attheir return to the crossing. Thus, new results on theinterference properties of wavepacket dynamics in such a systemwere obtained, such as the existence of two hybrid diabatic/adiabatic trajectories, robust towards thermal averaging.Further, we show that certain scanning possibility existbetween relative contents of these two trajectories at elevatedtemperature by scanning the pump wavelength. The systemrepresents a quantum matter-wave close analogue to an opticalpulsed Michelson interferometer. The dynamics of the A-statesystem was also investigated by anisotropy measurements. Thehigh degree of signal to noise ratio obtained, revealed a newtype of small oscillatory structure, which the analysis showsoriginates from coupling between all degrees of freedom of theRb2molecule, namely electronic, vibrational androtational motion.
The results of the work on the higher lying D-state systemconsist of the determination of a parallel excitationmechanism, where two wavepackets are simultaneously created intwo different electronic states. Further analysis showed thattheir future dynamics proceed essentially independently. Oneperforms adiabatic dynamics in a singleshelf-shapedstate, while the other goes throughcurve-crossings of somewhat weaker coupling strength thanintermediate. We propose the shape of the final, unknown,pump-probe states, guided by the quantum dynamical simulationstogether with the experimental data.
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Rubensson, Emanuel H. "Matrix Algebra for Quantum Chemistry." Doctoral thesis, Stockholm : Bioteknologi, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9447.
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Ying, Fuming. "Application and development of quantum chemical methods. Density functional theory and valence bond theory." Licentiate thesis, KTH, Teoretisk kemi, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-25033.
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This thesis deals with two disjoint subdiciplines of quantum chemistry. One isthe most used electronic structure method today, density functional theory(DFT), and the other one of the least used electronic structure methods,valence bond theory (VB). The work on DFT is based on previous developments inthe department in density functional response theory and involves studies ofhyperfine coupling constants which are measured in electron paramagneticresonance experiments. The method employed is a combination of arestricted-unrestriced approaches which allows for adequate description of spinpolarization without spin contamination, and spin-orbit corrections to accountfor heavy atom effects useing degenerate perturbation theory. The work anvalence bond theory is a new theoretical approach to higher-order derivatives.The orbital derivatives are complicated by the fact that the wave functions areconstructed from determinants of non-orthogonal orbitals. An approach based onnon-orthogonal second-quantization in biorthogonal basis sets leads tostraightforward derivations without explicit references to overlap matrices.These formulas are relevant for future applications in time-dependent valencebond theory.QC 20101006
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Rasmussen, Andrew Musso. "Theory of the Control of Ultrafast Interfacial Electron Transfer." Thesis, Northwestern University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3705348.
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This dissertation describes the theoretial exploration of electron transfer (ET) processes at the interface between bulk and molecular or nanoscale materials. Analysis of simple model Hamiltonians, those for the two- and three-level electronic systems as well as for a single electronic level coupled to a continuum, inform an understanding of electron transfer in nontrivial systems. A new treatment of the three-level system at an undergraduate level encapsulates the hopping and superexchange mechanisms of electron transfer. The elegance of the behavior of ET from a single-level/continuum system precedes a treatment of the reverse process—quasicontinuum-to-discrete level ET. This reverse process, relevant to ET from a bulk material to a semiconductor quantum dot (QD) offers a handle for the coherent control of ET at an interface: the shape of an electronic wavepacket within the quasicontinuum. An extension of the single-level-to-continuum ET process is the injection of an electron from a QD to a wide-bandgap semiconductor nanoparticle (NP). We construct a minimal model to explain trends in ET rates at the QD/NP interface as a function of QD size. Finally, we propose a scheme to gate ET through a molecular junction via the coherent control of the torsional mode(s) of a linking molecule within the junction.
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Lao, Ka Un. "Accurate and Efficient Quantum Chemistry Calculations for Noncovalent Interactions in Many-Body Systems." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1457973344.
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Abrams, Micah Lowell. "General-Order Single-Reference and Mulit-Reference Methods in Quantum Chemistry." Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/6852.
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Many-body perturbation theory and coupled-cluster theory, combined with carefully constructed basis sets, can be used to accurately compute the properties of small molecules. We applied a series of methods and basis sets aimed at reaching the ab initio limit to determine the barrier to planarity for ethylene cation. For potential energy surfaces corresponding to bond dissociation, a single Slater determinant is no longer an appropriate reference, and the single-reference hierarchy breaks down. We computed full configuration interaction benchmark data for calibrating new and existing quantum chemical methods for the accurate description of potential energy surfaces. We used the data to calibrate single-reference configuration interaction, perturbation theory, and coupled-cluster theory and multi-reference configuration interaction and perturbation theory, using various types of molecular orbitals, for breaking single and multiple bonds on ground-state and excited-state surfaces. We developed a determinant-based method which generalizes the formulation of many-body wave functions and energy expectation values. We used the method to calibrate single-reference and multi-reference configuration interaction and coupled-cluster theories, using different types of molecular orbitals, for the symmetric dissociation of water. We extended the determinant-based method to work with general configuration lists, enabling us to study, for the first time, arbitrarily truncated coupled-cluster wave functions. We used this new capability to study the importance of configurations in configuration interaction and coupled-cluster wave functions at different regions of a potential energy surface.
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Clarke, John Nicholas. "Applications of modern valence bond theory to small molecules." Thesis, University of Liverpool, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260246.
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Li, Ying. "Confined quantum fermionic systems." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28186.
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Thesis (M. S.)--Physics, Georgia Institute of Technology, 2009.Committee Chair: Landman, Uzi; Committee Member: Barnett, Robert; Committee Member: Chou, Meiyin; Committee Member: El-Sayed, Mostafa; Committee Member: Yannouleas, Constantine.
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Dinescu, Adriana. "Metals in Chemistry and Biology: Computational Chemistry Studies." Thesis, University of North Texas, 2007. https://digital.library.unt.edu/ark:/67531/metadc3678/.
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Numerous enzymatic reactions are controlled by the chemistry of metallic ions. This dissertation investigates the electronic properties of three transition metal (copper, chromium, and nickel) complexes and describes modeling studies performed on glutathione synthetase. (1) Copper nitrene complexes were computationally characterized, as these complexes have yet to be experimentally isolated. (2) Multireference calculations were carried out on a symmetric C2v chromium dimer derived from the crystal structure of the [(tBu3SiO)Cr(µ-OSitBu3)]2 complex. (3) The T-shaped geometry of a three-coordinate β-diketiminate nickel(I) complex with a CO ligand was compared and contrasted with isoelectronic and isosteric copper(II) complexes. (4) Glutathione synthetase (GS), an enzyme that belongs to the ATP-grasp superfamily, catalyzes the (Mg, ATP)-dependent biosynthesis of glutathione (GSH) from γ-glutamylcysteine and glycine. The free and reactant forms of human GS (wild-type and glycine mutants) were modeled computationally by employing molecular dynamics simulations, as these currently have not been structurally characterized.
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Rintelman, Jamie Marie. "Quantum Chemistry, and Eclectic Mix From Silicon Carbide to Size Consistency." Washington, D.C. : Oak Ridge, Tenn. : United States. Dept. of Energy. Office of Science ; distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy, 2004. http://www.osti.gov/servlets/purl/835379-0CKsBH/webviewable/.
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19 Dec 2004.Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1948" Jamie Marie Rintelman. 12/19/2004. Report is also available in paper and microfiche from NTIS.
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Jóhannesson, Gísli Hólmar. "Optimal hyperplanar transition state theory /." Thesis, Connect to this title online; UW restricted, 2000. http://hdl.handle.net/1773/11549.
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Rajagopal, Gunaretnam. "Systems of coupled quantum and classical degrees of freedom." Diss., Georgia Institute of Technology, 1991. http://hdl.handle.net/1853/30689.
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Zimmerman, Steven. "Hückel Energy of a Graph: Its Evolution From Quantum Chemistry to Mathematics." Master's thesis, University of Central Florida, 2011. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4729.
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The energy of a graph began with German physicist, Erich Hückel's 1931 paper, Quantenttheoretische Beiträge zum Benzolproblem. His work developed a method for computing the binding energy of the pi]-electrons for a certain class of organic molecules. The vertices of the graph represented the carbon atoms while the single edge between each pair of distinct vertices represented the hydrogen bonds between the carbon atoms. In turn, the chemical graphs were represented by an n x n matrix used in solving Schrödinger's eigenvalue/eigenvector equation. The sum of the absolute values of these graph eigenvalues represented the total pi]-electron energy. The criteria for constructing these chemical graphs and the chemical interpretations of all the quantities involved made up the Hückel Molecular Orbital theory or HMO theory. In this paper, we will show how the chemical interpretation of Hückel's graph energy evolved to a mathematical interpretation of graph energy that Ivan Gutman provided for us in his famous 1978 definition of the energy of a graph. Next, we will present Charles Coulson's 1940 theorem that expresses the energy of a graph as a contour integral and prove some of its corollaries. These corollaries allow us to order the energies of acyclic and bipartite graphs by the coefficients of their characteristic polynomial. Following Coulson's theorem and its corollaries we will look at McClelland's first theorem on the bounds for the energy of a graph. In the corollaries that follow McClelland's 1971 theorem, we will prove the corollaries that show a direct variation between the energy of a graph and the number of its vertices and edges. Finally, we will see how this relationship led to Gutman's conjecture that the complete graph on n vertices has maximal energy. Although this was disproved by Chris Godsil in 1981, we will provide an independent counterexample with the help of the software, Maple 13.ID: 030646262; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (M.S.)--University of Central Florida, 2011.; Includes bibliographical references (p. 32-34).
M.S.
Masters
Mathematics
Sciences
Mathematical Science
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Daver, Henrik. "Quantum Chemical Modeling of Phosphoesterase Mimics and Chemistry in Confined Spaces." Doctoral thesis, Stockholms universitet, Institutionen för organisk kemi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-148259.
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In this thesis, density functional theory is employed in the study of two kinds of systems that can be considered to be biomimetic in their own ways. First, three binuclear metal complexes, synthesized by the group of Prof. Ebbe Nordlander, have been investigated. The complexes are designed to resemble the active sites of phosphatase enzymes and have been examined in complexes where either two Zn(II) ions or one Fe(III) and one Mn(II) ion are bound. These dinuclear compounds were studied as catalysts for the hydrolysis of bis(2,4-dinitrophenyl) phosphate and the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate, which are model systems for the same reactions occurring in DNA or RNA. It was found that the two reactions take place in similar ways: a hydroxide ion that is terminally bound to one of the metal centers acts either as a nucleophile in the hydrolysis reaction or as a base in the transesterification. The leaving groups depart in an effectively concerted manner, and the formed catalyst-product complexes are predicted to be the resting states of the catalytic cycles. The rate-determining free energy barriers are identified from the catalyst-product complex in one catalytic cycle to the transition state of nucleophilic attack in the next. Another type of biomimetic modeling is made with an aim of imitating the conceptual features of selective binding of guests and screening them from solute-solvent interactions. Such features are found in so-called nanocontainers, and this thesis is concerned with studies of two capsules synthesized by the group of Prof. Julius Rebek, Jr. First, the cycloaddition of phenyl acetylene and phenyl azide has experimentally been observed to be accelerated in the presence of a capsule. Computational studies were herein performed on this system, and a previously unrecognized structure of the capsule is discovered. Two main factors are then identified as sources of the rate acceleration compared to the uncatalyzed reaction, namely the reduction of the entropic component and the selective destabilization of the reactant supercomplex over the transition state. In the second capsule study, the alkane binding trends of a water-soluble cavitand was studied. It is found that implicit solvation models fail severely in reproducing the experimental equilibrium observed between binding of n-decane by the cavitand monomer and encapsulation in the capsule dimer. A mixed explicit/implicit solvation protocol is developed to better quantify the effect of hydrating the cavitand, and a simple correction to the hydration free energy of a single water molecule is proposed to remedy this. The resulting scheme is used to predict new hydration free energies of the cavitand complexes, resulting in significant improvement vis-à-vis experiments. The computational results presented in this thesis show the usefulness of the quantum chemical calculations to develop understanding of experimental trends observed for substrate binding and catalysis. In particular, the methodology is shown to be versatile enough such that experimental observations can be reproduced for such diverse systems as studied herein.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.
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Borini, Stefano. "Theory and applications of advanced techniques in quantum chemistry and their integration in a common infrastructure." Toulouse 3, 2006. http://www.theses.fr/2006TOU30059.
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Crous, Werner. "The evaluation of the ONIOM-EE method for the QM/MM hybrid modeling of HF, CO and CO/HF Clusters." Thesis, Stellenbosch : Stellenbosch University, 2006. http://hdl.handle.net/10019.1/21774.
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Thesis (MSc)--University of Stellenbosch, 2006.ENGLISH ABSTRACT: Quantum mechanics is the method of choice when it comes to the accurate modeling of single molecules and clusters. The correlation energy is the single most important aspect when studying clusters computationally, and reproducing the correlation energy accurately poses a bigger challenge to the computational chemist than in the modeling of single molecules. Very high levels of theory and large basis sets need to be used. Nevertheless, since the calculation of large systems, such as crystals and biological systems, is generally beyond the capacity of quantum mechanics, molecular mechanics is generally used for these systems. Unfortunately due to its nature, molecular mechanics cannot model important quantum effects, but this problem can be solved by a hybrid system in which one part of the system is treated by quantum mechanics and the remaining part by molecular mechanics. In order to combine quantum mechanics with molecular mechanics one needs to optimize the parameters for the molecular mechanics part to allow it to function with the quantum mechanics. The research described in this work is based on the ONIOM-EE method, which is such a hybrid method. In this work we investigate the applicability of the ONIOM-EE method in modeling hydrogen fluoride, carbon monoxide and CO/HF clusters. Most of the clusters’ geometries in this work are not experimentally or computationally known. We therefore perform a computational analysis of all of the clusters by using various methods including Atoms in Molecules, Natural Bond Orbital analysis, Mulliken population analysis and the analysis of delocalized molecular orbitals to obtain information for the development of hybrid systems. During this process we look at different charge derivation schemes and at two different methods of optimizing force field parameters for these clusters. We develop a method to make force field optimization faster and better for specific hybrid systems. This method showed that in all cases the optimized parameters were an improvement on those of the Universal Force Field. We show the importance of an accurate description of the electrostatic interactions in HF, CO and CO/HF clusters and that this is the Achilles heel when attempting to optimize van der Waals parameters for force fields. We further show that atomic point charges are not a good approximation of a molecules’ charge density in hybrid methods. In addition, we make suggestions on how the present method for ONIOM-EE can be improved to make the modeling of van der Waals clusters feasible.
AFRIKAANSE OPSOMMING: Kwantum meganika is die metode van keuse wanneer enkele molekule en molekulêre sisteme op rekenaar gemodeleer moet word. Dit is egter bekend dat die modelering van molekulêre sisteme ’n groter uitdaging stel aan die molekulêre modeleerder, aangesien baie hoë vlakke van teorie en groot basisstelle gebruik moet word om die korrelasie-energie, rekenkundig te produseer. Die akkurate herprodusering van die korrelasie-energie is seker die heel belangrikste vereiste waaraan voldoen moet word as molekulêre sisteme d.m.v. ’n rekenaar gemodeleer word. Nietemin is dit onprakties om kwantum meganiese metodes te gebruik vir groot sisteme soos kristalle of biologiese molekule en juis om dié rede word molekulêre meganika meestal ingespan vir sulke gevalle. Molekulêre meganika is egter ondoeltreffend om belangrike kwantumeffekte te modeleer. Tog is daar ’n oplossing vir hierdie probleem in die vorm van ’n hibried sisteem waar een deel van die sisteem met kwantum meganika en die oorblywende deel van die sisteem met molekulêre meganika behandel word. Om dit moontlik te maak om molekulêre meganika met kwantum meganika te kombineer, moet parameters vir die molekulêre meganika deel geoptimiseer word sodat dit saam met die kwantum meganiese deel kan funksioneer. Die navorsing wat in hierdie studie beskryf word is gebaseer op so ’n hibriedmetode wat bekend staan as ONIOM-EE. In hierdie studie bestudeer ons die moontlikheid om ONIOM-EE te gebruik vir die modelering van molekulêre sisteme van waterstoffluoried, koolstofmonoksied en CO/HF sisteme. Die meeste van die sisteme, wat in hierdie studie behandel word, se strukture is onbekend, beide in terme van eksperimentele gegewens en molekulêre modelering. Ons voer dus ’n volledige analise van al die sisteme uit deur van verskeie metodes soos “Atoms in Molecules”, “Natural Bond Orbital” analise, Mulliken populasie analise en die analise van gedelokaliseerde molekulêre orbitale, gebruik te maak. Dit stel ons in staat om ’n hibriedsisteem te ontwikkel vir die molekulêre sisteme. Gedurende die proses ondersoek ons ook die gebruik van verskillende ladingsafleidings-sisteme en twee metodes word ondersoek waarop ’n kragveld vir ’n hibriedsisteem geoptimiseer kan word. Ons toon aan dat die geoptimiseerde parameters beter resultate lewer as die van die “Universal Force Field” en lig ook die belangrikheid daarvan uit dat die elektrostatiese interaksies se beskrywing ’n hibriedsisteem se Achilles hiel is indien van der Waals parameters geoptimiseer moet word. Ons toon aan dat die gebruik van puntladings op atome om die ladingsdigtheid in molekulêre sisteme te beskryf, ’n onakkurate benadering is. Sekere aanbevelings hoe om die ONIOM-EE metode sodanig te verbeter, dat dit wel gebruik kan word om van der Waals sisteme suksesvol te modeleer, word ook gemaak.
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Puzder, Aaron. "A quantum Monte Carlo study of exchange and correlation in the silicon pseudo atom." Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/30713.
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Loper, Robert D. "Collisional broadening and shift of D1 and D2 spectral lines in atomic alkali vapor - noble gas systems." Thesis, Air Force Institute of Technology, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3556522.
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The Baranger model is used to compute collisional broadening and shift of the D1 and D2 spectral lines of M + Ng, where M = K, Rb, Cs and Ng = He, Ne, Ar, using scattering phase shift differences which are calculated from scattering matrix elements. Scattering matrix elements are calculated using the Channel Packet Method where the collisions are treated non-adiabatically and include spin-orbit and Coriolis couplings. Non-adiabatic wavepacket dynamics are determined using the split-operator method together with a unitary transformation between adiabatic and diabatic representations. Scattering phase shift differences are thermally weighted and integrated over energies ranging from E = 0 Hartree up to E = 0.0075 Hartree and averaged over values of total angular momentum that range from J = 0.5 up to J = 400.5. Phase shifts are extrapolated linearly to provide an approximate extension of the energy regime up to E = 0.012 Hartree. Broadening and shift coefficients are obtained for temperatures ranging from T = 100 K up to T = 800 K and compared with experiment. Predictions from this research find application in laser physics and specifically with improvement of total power output of Optically Pumped Alkali Laser systems.
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Wong, Kim Fay. "Refinement of hybrid mixed quantum-classical methodology for chemical dynamics in solutions and molecular materials /." Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3064688.
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Olander, Jenny. "Growth studies of SiC and BN : from theory and experiments /." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3439.
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Jarrold, Thomas Furnley. "Single channel Kondo physics in triple quantum dots." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:e2772c4e-6c76-44b8-9c02-401d9f90b27f.
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In this thesis we investigate a system of three tunnel-coupled quantum dots, arranged in a triangular geometry and attached to a single metallic conduction band, using both analytic and semi-analytic methods and the numerical renormalisation group technique. This is the simplest coupled quantum dot system to exhibit frustration. We study three different models of the triple quantum dot device: a mirror symmetric arrangement of dots in which only one dot is connected to the conduction band, a triple quantum dot system in which only one dot is connected to the conduction band without a plane of mirror symmetry and a mirror symmetric arrangement of quantum dots in which all three dots are coupled to the conduction band. We study these models over a wide range of parameter space, and in both the presence and absence of a magnetic field. Both antiferromagnetic and ferromagnetic Kondo effects are observed, and in all three models we find that the system contains at least two phases, and so a number of quantum phase transitions may be observed, associated in some cases with significant changes in the low temperature conductance through the triple quantum dot device. In addition to zero-field Kondo physics, a number of field induced Kondo effects are also observed. Both first order quantum phase transitions and Kosterlitz-Thouless phase transitions are observed. We use both symmetry arguments and low energy effective models which we derive to explain and understand both the position of and type of phase boundary that is observed in each case, and perturbative methods are used to accurately predict Kondo temperatures for a wide range of systems.
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Ghebreysus, Woldu Mengistu. "Quantum mechanical and experimental infra-red studies on stability and structural properties of substituted acylthiourea compounds." Thesis, Stellenbosch : Stellenbosch University, 2004. http://hdl.handle.net/10019.1/50073.
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Hohenstein, Edward G. "Implementation and applications of density-fitted symmetry-adapted perturbation theory." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42699.
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Noncovalent interactions play a vital role throughout much of chemistry. The understanding and characterization of these interactions is an area where theoretical chemistry can provide unique insight. While many methods have been developed to study noncovalent interactions, symmetry-adapted perturbation theory (SAPT) stands out as one of the most robust. In addition to providing energetic information about an interaction, it provides insight into the underlying physics of the interaction by decomposing the energy into electrostatics, exchange, induction and dispersion. Therefore, SAPT is capable of not only answering questions about how strongly a complex is bound, but also why it is bound. This proves to be an invaluable tool for the understanding of noncovalent interactions in complex systems.
The wavefunction-based formulation of SAPT can provide qualitative results for large systems as well as quantitative results for smaller systems. In order to extend the applicability of this method, approximations to the two-electron integrals must be introduced. At low-order, the introduction of density fitting approximations allows SAPT computations to be performed on systems with up to 220 atoms and 2850 basis functions. Higher-orders of SAPT, which boasts accuracy rivaling the best theoretical methods, can be applied to systems with over 40 atoms. Higher-order SAPT also benefits from approximations that attempt to truncate unneccesary unoccupied orbitals.
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Sharkey, Keeper Layne. "Very Accurate Quantum Mechanical Non-Relativistic Spectra Calculations of Small Atoms & Molecules Employing All-Particle Explicitly Correlated Gaussian Basis Functions." Diss., The University of Arizona, 2015. http://hdl.handle.net/10150/560835.
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Due to the fast increasing capabilities of modern computers it is now feasible to calculate spectra of small atom and molecules with the greater level of accuracy than high-resolution measurements. The mathematical algorithms developed and implemented on high performance supercomputers for the quantum mechanical calculations are directly derived from the first principles of quantum mechanics. The codes developed are primarily used to verify, refine, and predict the energies associated within a given system and given angular momentum state of interest. The Hamiltonian operator used to determine the total energy in the approach presented is called the internal Hamiltonian and is obtained by rigorously separating out the center-of-mass motion (or the elimination of translational motion) from the laboratory-frame Hamiltonian. The methods utilized in the articles presented in this dissertation do not include relativistic corrections and quantum electrodynamic effects, nor do these articles assume the Born-Oppenheimer (BO) approximation with the exception of one publication. There is one major review article included herein which describes the major differences between the non-BO method and the BO approximation using explicitly correlated Gaussian (ECG) basis functions. The physical systems studied in this dissertation are the atomic elements with Z < 7 (although the discussion is not limited to these) and diatomic molecules such as H₂⁺ and H₂ including nuclear isotopic substitution studies with deuterium and tritium, as well as electronic substitutions with the muon particle. Preliminary testing for triatomic molecular functionals using a model potential is also included in this dissertation. It has been concluded that using all-particle ECGs with including the addition of nonzero angular momentum functions to describe nonzero angular momentum states is sufficient in determining the energies of these states for both the atomic and molecular case.
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Lange, Adrian W. "Multi-layer Methods for Quantum Chemistry in the Condensed Phase: Combining Density Functional Theory, Molecular Mechanics, and Continuum Solvation Models." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1329752615.
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Jayatilaka, Frederic William. "Theoretical studies of tunnel-coupled double quantum dots." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:756add23-aae6-4a71-a22b-087695bc600a.
Full textAbstract:
We study the low-temperature physics arising in models of a strongly correlated, tunnel-coupled double quantum dot (DQD), particularly the two-impurity Anderson model (2AIM) and the two-impurity Kondo model (2IKM), employing a combination of physical arguments and the Numerical Renormalisation Group. These models exhibit a rich range of Kondo physics. In the regime with essentially one electron on each dot, there is a competition between the Kondo effect and the interdot exchange interaction. This competition gives rise to a quantum phase transition (QPT) between local singlet and Kondo singlet phases in the 2IKM, which becomes a continuous crossover in the 2AIM as a result of the interlead charge transfer present. The 2IKM is known to exhibit two-channel Kondo (2CK) physics at the QPT, and we investigate whether this is also the case for the 2AIM at the crossover. We find that while in principle 2CK physics can be observed in the 2AIM, extremely low temperatures are required, such that it is unlikely that 2CK physics will be observed in an experimental DQD system in the near future. We have studied the effect of a magnetic field on the 2AIM and the 2IKM, finding that both the zero-field QPT in the 2IKM and the zero-field crossover in the 2AIM, persist to finite field. This presents the possibility of observing 2CK physics in an experimental DQD at finite field, but we find that the temperatures required to do so are extremely low. We show that longer even-numbered chains of spins also exhibit QPTs at finite field, and argue that a 2N-spin chain should undergo N QPTs as field is increased (starting deep in the local singlet phase at zero field). We have also carried out a joint theoretical-experimental study of a carbon nanotube based DQD, in collaboration with Dr. Mark Buitelaar et al. The agreement between experimental and theoretical results is good, and the experiments are able to access the crossover present in the 2AIM at finite field. Furthermore, the experiments show the wide range of physics exhibited by DQD systems, and illustrate the utility of such systems in probing correlated electron physics.
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Wåhlin, Pernilla. "Theoretical Actinide Chemistry – Methods and Models." Doctoral thesis, Stockholms universitet, Fysikum, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-54848.
Full textAbstract:
The chemistry of actinides in aqueous solution is important, and it is essential to build adequate conceptual models and develop methods applicable for actinide systems. The complex electronic structure makes benchmarking necessary. In the thesis a prototype reaction of the water exchange reaction for uranyl(VI), for both ground and luminescent states, described with a six-water model, was used to study the applicability of density functional methods on actinides and different solvation models. An excellent agreement between the wave function methods CCSD(T) and MP2 was obtained in the ground state, implying that near-minimal CASPT2 can be used with confidence for the reaction in the luminescent state of uranyl(VI), while density functionals are not suited to describe energetics for this type of reaction. There was an ambiguity concerning the position of the waters in the second hydration sphere. This issue was resolved by investigating a larger model, and prop- erly used the six-water model was found to adequately describe the water exchange reaction. The effect of solvation was investigated by comparing the results from conductor-like polarizable continuum models using two cavity models. Scattered numbers made it difficult to determine which solvation model to use. The final conclusion was that the water exchange reaction in the luminescent state of uranyl(VI) should be addressed with near-minimal CASPT2 and a solvation model without explicit cavities for hydrogens. Finally it was shown that no new chemistry appears in the luminescent state for this reaction. The thesis includes a methodological investigation of a multi-reference density functional method based on a range separation of the two-electron interaction. The method depends on a universal parameter, which has been determined for lighter elements. It is shown here that the same parameter could be used for actinides, a prerequisite for further development of the method. The results are in that sense promising.
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Schwarz, Lauretta Rebecca. "Projector Quantum Monte Carlo methods for linear and non-linear wavefunction ansatzes." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/267871.
Full textAbstract:
This thesis is concerned with the development of a Projector Quantum Monte Carlo method for non-linear wavefunction ansatzes and its application to strongly correlated materials. This new approach is partially inspired by a prior application of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) method to the three-band (p-d) Hubbard model. Through repeated stochastic application of a projector FCIQMC projects out a stochastic description of the Full Configuration Interaction (FCI) ground state wavefunction, a linear combination of Slater determinants spanning the full Hilbert space. The study of the p-d Hubbard model demonstrates that the nature of this FCI expansion is profoundly affected by the choice of single-particle basis. In a counterintuitive manner, the effectiveness of a one-particle basis to produce a sparse, compact and rapidly converging FCI expansion is not necessarily paralleled by its ability to describe the physics of the system within a single determinant. The results suggest that with an appropriate basis, single-reference quantum chemical approaches may be able to describe many-body wavefunctions of strongly correlated materials. Furthermore, this thesis presents a reformulation of the projected imaginary time evolution of FCIQMC as a Lagrangian minimisation. This naturally allows for the optimisation of polynomial complex wavefunction ansatzes with a polynomial rather than exponential scaling with system size. The proposed approach blurs the line between traditional Variational and Projector Quantum Monte Carlo approaches whilst involving developments from the field of deep-learning neural networks which can be expressed as a modification of the projector. The ability of the developed approach to sample and optimise arbitrary non-linear wavefunctions is demonstrated with several classes of Tensor Network States all of which involve controlled approximations but still retain systematic improvability towards exactness. Thus, by applying the method to strongly-correlated Hubbard models, as well as ab-initio systems, including a fully periodic ab-initio graphene sheet, many-body wavefunctions and their one- and two-body static properties are obtained. The proposed approach can handle and simultaneously optimise large numbers of variational parameters, greatly exceeding those of alternative Variational Monte Carlo approaches.
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Tucker, Adam Philip. "Local moment phases in quantum impurity problems." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:538d2d83-963e-4a51-81cd-4235e9761da4.
Full textAbstract:
This thesis considers quantum impurity models that exhibit a quantum phase transition (QPT) between a Fermi liquid strong coupling (SC) phase, and a doubly-degenerate non-Fermi liquid local moment (LM) phase. We focus on what can be said from exact analytic arguments about the LM phase of these models, where the system is characterized by an SU(2) spin degree of freedom in the entire system. Conventional perturbation theory about the non-interacting limit does not hold in the non-Fermi liquid LM phase. We circumvent this problem by reformulating the perturbation theory using a so-called `two self-energy' (TSE) description, where the two self-energies may be expressed as functional derivatives of the Luttinger-Ward functional. One particular paradigmatic model that possesses a QPT between SC and LM phases is the pseudogap Anderson impurity model (PAIM). We use infinite-order perturbation theory in the interaction, U, to self-consistently deduce the exact low-energy forms of both the self-energies and propagators in each of the distinct phases of the model. We analyse the behaviour of the model approaching the QPT from each phase, focusing on the scaling of the zero-field single-particle dynamics using both analytical arguments and detailed numerical renormalization group (NRG) calculations. We also apply two `conserving' approximations to the PAIM. First, second-order self-consistent perturbation theory and second, the fluctuation exchange approximation (FLEX). Within the FLEX approximation we develop a numerical algorithm capable of self-consistently and coherently describing the QPT coming from both distinct phases. Finally, we consider a range of static spin susceptibilities that each probe the underlying QPT in response to coupling to a magnetic field.
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Mordovina, Uliana [Verfasser], and Angel [Akademischer Betreuer] Rubio. "Novel Approaches in Quantum Chemistry : Self-Consistent Density-Functional Embedding and Polaritonic Coupled-Cluster Theory / Uliana Mordovina ; Betreuer: Angel Rubio." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2020. http://d-nb.info/1210647176/34.
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Wright, Christopher James. "Theoretical studies of underscreened Kondo physics in quantum dots." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:62207edb-af3a-4340-a6f2-5264b1374a41.
Full textAbstract:
We study correlated two-level quantum impurity models coupled to a metallic conduction band in the hope of gaining insight into the physics of nanoscale quantum dot systems. We focus on the possibility of formation of a spin-1 impurity local moment which, on coupling to the band, generates an underscreened (USC) singular Fermi liquid state. By employing physical arguments and the numerical renormalization group (NRG) technique, we analyse such systems in detail examining in particular both the thermodynamic and dynamic properties, including the differential conductance. The quantum phase transitions occurring between the USC phase and a more ordinary Fermi liquid (FL) phase are analysed in detail. They are generically found to be of Kosterlitz-Thouless type; exceptions occur along lines of high symmetry where first-order transitions are found. A `Friedel-Luttinger sum rule' is derived and, together with a generalization of Luttinger's theorem to the USC phase, is used to obtain general results for the $T=0$ zero-bias conductance --- it is expressed solely in terms of the number of electrons present on the impurity and applicable in both the USC and FL phases. Relatedly, dynamical signatures of the quantum phase transition show two broad classes of behaviour corresponding to the collapse of either a resonance or antiresonance in the single-particle density of states. Evidence of both of these behaviours is seen in experimental devices. We study also the effect of a local magnetic field on both single- and two-level quantum impurities. In the former case we attempt to resolve some points of contention that remain in the literature. Specifically we show that the position of the maximum in the spin resolved density of states (and related peaks in the differential conductance) is not linear in the applied field, showing a more complicated form than a simple `Zeeman splitting'. The analytic result for the low-field asymptote is recovered. For two-level impurities we illustrate the manner in which the USC state is destroyed: due to two cancelling effects an abrupt change in the zero-bias conductance does not occur as one might expect. Comparison with experiment is made in both cases and used to interpret experimental findings in a manner contrary to previous suggestions. We find that experiments are very rarely in the limit of strong impurity-host coupling. Further, features in the differential conductance as a function of bias voltage should not be simply interpreted in terms of isolated quantum dot states. The many-body nature of such systems is crucially important to their observed properties.
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Acheampong, Edward. "Computational Quantum Chemistry Studies of the Interactions of Amino Acids Side Chains with the Guanine Radical Cation." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etd/3489.
Full textAbstract:
Guanine is generally accepted as the most easily oxidized DNA base when cells are subjected to ionizing radiation, photoionization or photosensitization. At pH 7, the midpoint reduction potential is on the order of 0.2 – 0.3 V higher than those of the radicals of e.g. tyrosine, tryptophan cysteine and histidine, so that the radical “repair” (or at least, a thermodynamically favorable reaction) involving these amino acids is feasible. Computational quantum studies have been done on tyrosine, tryptophan, cysteine and histidine side chains as they appear in histones. Density functional theory was employed using B3LYP/6-31G+ (d, p) basis set to study spin densities on these amino acids side chains as they pair with the guanine radical cation. The amino acid side chains are positioned so as not to disrupt the Watson-Crick base pairing. Our results indicate that, these side chains of amino acid with reducing properties can repair guanine radical cation through electron transfer coupled with proton transfer.
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Lundell, Sandra J. "Quantum Mechanical Studies of N-H···N Hydrogen Bonding in Acetamide Derivatives and Amino Acids." DigitalCommons@USU, 2018. https://digitalcommons.usu.edu/etd/7309.
Full textAbstract:
Proteins are made of vast chains of amino acids that twist and fold into intricate designs. These structures are held in place by networks of noncovalent interactions. One of these, the hydrogen bond, forms bridges between adjacent pieces of the protein chain and is one of the most important contributors to the shape and stability of proteins. Hydrogen bonds come in all shapes and sizes and a full understanding of these not only aids in our understanding of proteins in general but can bridge the gap to finding cures to many protein-related diseases, such as sickle-cell anemia. The primary aim of this thesis is to discover if a specific type of hydrogen bond, the N-H···N bond, occurs within proteins and if so, if it contributes to the structure and stability of proteins.
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Jacobson, Leif David. "Approximating Many-Body Induction to Efficiently Describe Molecular Liquids and Clusters With Improved Accuracy." The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1312480919.
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Haworth, Naomi Louise. "Quantum Chemical Studies of Thermochemistry, Kinetics and Molecular Structure." Thesis, The University of Sydney, 2003. http://hdl.handle.net/2123/509.
Full textAbstract:
This thesis is concerned with a range of chemical problems which are amenable to theoretical investigation via the application of current methods of computational quantum chemistry. These problems include the calculation of accurate thermochemical data, the prediction of reaction kinetics, the study of molecular potential energy surfaces, and the investigation of molecular structures and binding. The heats of formation (from both atomisation energies and isodesmic schemes) of a set of approximately 120 C1 and C2 fluorocarbons and oxidised fluorocarbons (along with C3F6 and CF3CHFCF2) were calculated with the Gaussian-3 (G3) method (along with several approximations thereto). These molecules are of importance in the flame chemistry of 2-H-heptafluoropropane, which has been proposed as a potential fire retardant with which to replace chloro- and bromofluorocarbons (CFC�s and BFC�s). The calculation of the data reported here was carried out in parallel with the modelling studies of Hynes et al.1-3 of shock tube experiments on CF3CHFCF3 and on C3F6 with either hydrogen or oxygen atoms. G3 calculations were also employed in conjunction with the experimental work of Owens et al.4 to describe the pyrolysis of CFClBr2 (giving CFCl) at a radiation wavelength of 265 nm. The theoretical prediction of the dissociation energy of the two C-Br bonds was found to be consistent with the energy at which carbene production was first observed, thus supporting the hypothesis that the pyrolysis releases two bromine radicals (rather than a Br2 molecule). On the basis of this interpretation of the experiments, the heat of formation of CFClBr2 is predicted to be 184 � 5 kJ mol-1, in good agreement with the G3 value of 188 � 5 kJ mol-1. Accurate thermochemical data was computed for 18 small phosphorus containing molecules (P2, P4, PH, PH2, PH3, P2H2, P2H4, PO, PO2, PO3, P2O, P2O2, HPO, HPOH, H2POH, H3PO, HOPO and HOPO2), most of which are important in the reaction model introduced by Twarowski5 for the combustion of H2 and O2 in the presence of phosphine. Twarowski reported that the H + OH recombination reaction is catalysed by the combustion products of PH3 and proposed two catalytic cycles, involving PO2, HOPO and HOPO2, to explain this observation. Using our thermochemical data we computed the rate coefficients of the most important reactions in these cycles (using transition state and RRKM theories) and confirmed that at 2000K both cycles have comparable rates and are significantly faster than the uncatalysed H + OH recombination. The heats of formation used in this work on phosphorus compounds were calculated using the G2, G3, G3X and G3X2 methods along with the far more extensive CCSD(T)/CBS type scheme. The latter is based on the evaluation of coupled cluster energies using the correlation consistent triple-, quadruple- and pentuple-zeta basis sets and extrapolation to the complete basis set (CBS) limit along with core-valence correlation corrections (with counterpoise corrections for phosphorus atoms), scalar relativistic corrections and spin-orbit coupling effects. The CCSD(T)/CBS results are consistent with the available experimental data and therefore constitute a convenient set of benchmark values with which to compare the more approximate Gaussian-n results. The G2 and G3 methods were found to be of comparable accuracy, however both schemes consistently underestimated the benchmark atomisation energies. The performance of G3X is significantly better, having a mean absolute deviation (MAD) from the CBS results of 1.8 kcal mol-1, although the predicted atomisation energies are still consistently too low. G3X2 (including counterpoise corrections to the core-valence correlation energy for phosphorus) was found to give a slight improvement over G3X, resulting in a MAD of 1.5 kcal mol-1. Several molecules were also identified for which the approximations underlying the Gaussian-n methodologies appear to be unreliable; these include molecules with multiple or strained P-P bonds. The potential energy surface of the NNH + O system was investigated using density functional theory (B3LYP/6-31G(2df,p)) with the aim of determining the importance of this route in the production of NO in combustion reactions. In addition to the standard reaction channels, namely decomposition into NO + NH, N2 + OH and H + N2O via the ONNH intermediate, several new reaction pathways were also investigated. These include the direct abstraction of H by O and three product channels via the intermediate ONHN, giving N2 + OH, H + N2O and HNO + N. For each of the species corresponding to stationary points on the B3LYP surface, valence correlated CCSD(T) calculations were performed with the aug-cc-pVxZ (x = Q, 5) basis sets and the results extrapolated to the complete basis set limit. Core-valence correlation corrections, scalar relativistic corrections and spin orbit effects were also included in the resulting energetics and the subsequent calculation of thermochemical data. Heats of formation were also calculated using the G3X method. Variational transition state theory was used to determine the critical points for the barrierless reactions and the resulting B3LYP energetics were scaled to be compatible with the G3X and CCSD(T)/CBS values. As the results of modelling studies are critically dependent on the heat of formation of NNH, more extensive CCSD(T)/CBS calculations were performed for this molecule, predicting the heat of formation to be 60.6 � 0.5 kcal mol-1. Rate coefficients for the overall reaction processes were obtained by the application of multi-well RRKM theory. The thermochemical and kinetic results thus obtained were subsequently used in conjunction with the GRIMech 3.0 reaction data set in modelling studies of combustion systems, including methane / air and CO / H2 / air mixtures in completely stirred reactors. This study revealed that, contrary to common belief, the NNH + O channel is a relatively minor route for the production of NO. The structure of the inhibitor Nd-(N'-Sulfodiaminophosphinyl)-L-ornithine, PSOrn, and the nature of its binding to the OTCase enzyme was investigated using density functional (B3LYP) theory. The B3LYP/6-31G(d) calculations on the model compound, PSO, revealed that, while this molecule could be expected to exist in an amino form in the gas phase, on complexation in the active site of the enzyme it would be expected to lose two protons to form a dinegative imino tautomer. This species is shown to bind strongly to two H3CNHC(NH2)2+ moieties (model compounds for arginine residues), indicating that the strong binding observed between inhibitor and enzyme is partially due to electrostatic interactions as well as extensive hydrogen bonding (both model Arg+ residues form hydrogen bonds to two different sites on PSO). Population analysis and hydrogen bonding studies have revealed that the intramolecular bonding in this species consists of either single or semipolar bonds (that is, S and P are not hypervalent) and that terminal oxygens (which, being involved in semipolar bonds, carry negative charges) can be expected to form up to 4 hydrogen bonds with residues in the active site. In the course of this work several new G3 type methods were proposed, including G3MP4(SDQ) and G3[MP2(Full)], which are less expensive approximations to G3, and G3X2, which is an extension of G3X designed to incorporate additional electron correlation. As noted earlier, G3X2 shows a small improvement on G3X; G3MP4(SDQ) and G3[MP2(Full)], in turn, show good agreement with G3 results, with MAD�s of ~ 0.4 and ~ 0.5 kcal mol-1 respectively. 1. R. G. Hynes, J. C. Mackie and A. R. Masri, J. Phys. Chem. A, 1999, 103, 5967. 2. R. G. Hynes, J. C. Mackie and A. R. Masri, J. Phys. Chem. A, 1999, 103, 54. 3. R. G. Hynes, J. C. Mackie and A. R. Masri, Proc. Combust. Inst., 2000, 28, 1557. 4. N. L. Owens, Honours Thesis, School of Chemistry, University of Sydney, 2001. 5. A. Twarowski, Combustion and Flame, 1995, 102, 41.
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Haworth, Naomi Louise. "Quantum Chemical Studies of Thermochemistry, Kinetics and Molecular Structure." University of Sydney. Chemistry, 2003. http://hdl.handle.net/2123/509.
Full textAbstract:
This thesis is concerned with a range of chemical problems which are amenable to theoretical investigation via the application of current methods of computational quantum chemistry. These problems include the calculation of accurate thermochemical data, the prediction of reaction kinetics, the study of molecular potential energy surfaces, and the investigation of molecular structures and binding. The heats of formation (from both atomisation energies and isodesmic schemes) of a set of approximately 120 C1 and C2 fluorocarbons and oxidised fluorocarbons (along with C3F6 and CF3CHFCF2) were calculated with the Gaussian-3 (G3) method (along with several approximations thereto). These molecules are of importance in the flame chemistry of 2-H-heptafluoropropane, which has been proposed as a potential fire retardant with which to replace chloro- and bromofluorocarbons (CFC�s and BFC�s). The calculation of the data reported here was carried out in parallel with the modelling studies of Hynes et al.1-3 of shock tube experiments on CF3CHFCF3 and on C3F6 with either hydrogen or oxygen atoms. G3 calculations were also employed in conjunction with the experimental work of Owens et al.4 to describe the pyrolysis of CFClBr2 (giving CFCl) at a radiation wavelength of 265 nm. The theoretical prediction of the dissociation energy of the two C-Br bonds was found to be consistent with the energy at which carbene production was first observed, thus supporting the hypothesis that the pyrolysis releases two bromine radicals (rather than a Br2 molecule). On the basis of this interpretation of the experiments, the heat of formation of CFClBr2 is predicted to be 184 � 5 kJ mol-1, in good agreement with the G3 value of 188 � 5 kJ mol-1. Accurate thermochemical data was computed for 18 small phosphorus containing molecules (P2, P4, PH, PH2, PH3, P2H2, P2H4, PO, PO2, PO3, P2O, P2O2, HPO, HPOH, H2POH, H3PO, HOPO and HOPO2), most of which are important in the reaction model introduced by Twarowski5 for the combustion of H2 and O2 in the presence of phosphine. Twarowski reported that the H + OH recombination reaction is catalysed by the combustion products of PH3 and proposed two catalytic cycles, involving PO2, HOPO and HOPO2, to explain this observation. Using our thermochemical data we computed the rate coefficients of the most important reactions in these cycles (using transition state and RRKM theories) and confirmed that at 2000K both cycles have comparable rates and are significantly faster than the uncatalysed H + OH recombination. The heats of formation used in this work on phosphorus compounds were calculated using the G2, G3, G3X and G3X2 methods along with the far more extensive CCSD(T)/CBS type scheme. The latter is based on the evaluation of coupled cluster energies using the correlation consistent triple-, quadruple- and pentuple-zeta basis sets and extrapolation to the complete basis set (CBS) limit along with core-valence correlation corrections (with counterpoise corrections for phosphorus atoms), scalar relativistic corrections and spin-orbit coupling effects. The CCSD(T)/CBS results are consistent with the available experimental data and therefore constitute a convenient set of benchmark values with which to compare the more approximate Gaussian-n results. The G2 and G3 methods were found to be of comparable accuracy, however both schemes consistently underestimated the benchmark atomisation energies. The performance of G3X is significantly better, having a mean absolute deviation (MAD) from the CBS results of 1.8 kcal mol-1, although the predicted atomisation energies are still consistently too low. G3X2 (including counterpoise corrections to the core-valence correlation energy for phosphorus) was found to give a slight improvement over G3X, resulting in a MAD of 1.5 kcal mol-1. Several molecules were also identified for which the approximations underlying the Gaussian-n methodologies appear to be unreliable; these include molecules with multiple or strained P-P bonds. The potential energy surface of the NNH + O system was investigated using density functional theory (B3LYP/6-31G(2df,p)) with the aim of determining the importance of this route in the production of NO in combustion reactions. In addition to the standard reaction channels, namely decomposition into NO + NH, N2 + OH and H + N2O via the ONNH intermediate, several new reaction pathways were also investigated. These include the direct abstraction of H by O and three product channels via the intermediate ONHN, giving N2 + OH, H + N2O and HNO + N. For each of the species corresponding to stationary points on the B3LYP surface, valence correlated CCSD(T) calculations were performed with the aug-cc-pVxZ (x = Q, 5) basis sets and the results extrapolated to the complete basis set limit. Core-valence correlation corrections, scalar relativistic corrections and spin orbit effects were also included in the resulting energetics and the subsequent calculation of thermochemical data. Heats of formation were also calculated using the G3X method. Variational transition state theory was used to determine the critical points for the barrierless reactions and the resulting B3LYP energetics were scaled to be compatible with the G3X and CCSD(T)/CBS values. As the results of modelling studies are critically dependent on the heat of formation of NNH, more extensive CCSD(T)/CBS calculations were performed for this molecule, predicting the heat of formation to be 60.6 � 0.5 kcal mol-1. Rate coefficients for the overall reaction processes were obtained by the application of multi-well RRKM theory. The thermochemical and kinetic results thus obtained were subsequently used in conjunction with the GRIMech 3.0 reaction data set in modelling studies of combustion systems, including methane / air and CO / H2 / air mixtures in completely stirred reactors. This study revealed that, contrary to common belief, the NNH + O channel is a relatively minor route for the production of NO. The structure of the inhibitor Nd-(N'-Sulfodiaminophosphinyl)-L-ornithine, PSOrn, and the nature of its binding to the OTCase enzyme was investigated using density functional (B3LYP) theory. The B3LYP/6-31G(d) calculations on the model compound, PSO, revealed that, while this molecule could be expected to exist in an amino form in the gas phase, on complexation in the active site of the enzyme it would be expected to lose two protons to form a dinegative imino tautomer. This species is shown to bind strongly to two H3CNHC(NH2)2+ moieties (model compounds for arginine residues), indicating that the strong binding observed between inhibitor and enzyme is partially due to electrostatic interactions as well as extensive hydrogen bonding (both model Arg+ residues form hydrogen bonds to two different sites on PSO). Population analysis and hydrogen bonding studies have revealed that the intramolecular bonding in this species consists of either single or semipolar bonds (that is, S and P are not hypervalent) and that terminal oxygens (which, being involved in semipolar bonds, carry negative charges) can be expected to form up to 4 hydrogen bonds with residues in the active site. In the course of this work several new G3 type methods were proposed, including G3MP4(SDQ) and G3[MP2(Full)], which are less expensive approximations to G3, and G3X2, which is an extension of G3X designed to incorporate additional electron correlation. As noted earlier, G3X2 shows a small improvement on G3X; G3MP4(SDQ) and G3[MP2(Full)], in turn, show good agreement with G3 results, with MAD�s of ~ 0.4 and ~ 0.5 kcal mol-1 respectively. 1. R. G. Hynes, J. C. Mackie and A. R. Masri, J. Phys. Chem. A, 1999, 103, 5967. 2. R. G. Hynes, J. C. Mackie and A. R. Masri, J. Phys. Chem. A, 1999, 103, 54. 3. R. G. Hynes, J. C. Mackie and A. R. Masri, Proc. Combust. Inst., 2000, 28, 1557. 4. N. L. Owens, Honours Thesis, School of Chemistry, University of Sydney, 2001. 5. A. Twarowski, Combustion and Flame, 1995, 102, 41.
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Mitchell, Andrew Keith. "Two-channel Kondo phases in coupled quantum dots." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:3d4e9d86-794c-441c-9d4b-20e6f1bd1de1.
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We investigate systems comprising chains and rings of quantum dots, coupled to two metallic leads. Such systems allow to study the competition between orbital and spin degrees of freedom in a nanodevice, and the effect this subtle interplay has on two-channel Kondo (2CK) physics. We demonstrate that a rich range of strongly correlated electron behaviour results, with non-Fermi liquid 2CK phases and non-trivial phase transitions accessible. We employ physical arguments and the numerical renormalization group (NRG) technique to analyse these systems in detail, examining in particular both thermodynamic and dynamical properties. When leads are coupled to either end of a chain of dots, we show that the resulting behaviour on low temperature/energy scales can be understood in terms of simpler paradigmatic quantum `impurity' models. An effective low-energy single-spin 2CK model is derived for all odd-length chains, while the behaviour of even-length chains is related fundamentally to that of the classic `two-impurity Kondo' model. In particular, for small interdot coupling, we show that an effective coupling mediated though incipient single-channel Kondo states drives all odd chains to the 2CK fixed point (FP) on the lowest temperature/energy scales. A theory is also developed to describe a phase transition in even chains. We derive an effective channel-anisotropic 2CK model, which indicates that the critical FP of such models must be the 2CK FP. This physical picture is confirmed using NRG for various chain systems. We also examine the effect of local frustration on 2CK physics in mirror-symmetric ring systems. The importance of geometry and symmetry is demonstrated clearly in the markedly different physical behaviour that arises in systems where two leads are either connected to the same dot, or to neighbouring dots. In the latter case, we show for all odd-membered rings that two distinct 2CK phases, with different ground state parities, arise on tuning the interdot couplings. A frustration-induced phase transition thus occurs, the 2CK phases being separated by a novel critical point for which an effective low-energy model is derived. Precisely at the transition, parity mixing of the quasidegenerate local trimer states acts to destabilise the 2CK FPs, and the critical FP is shown to consist of a free pseudospin together with effective single-channel spin quenching. While connecting both leads to the same dot again results in two parity-distinct phases, a simple level-crossing transition now results due to the symmetry of the setup. The proposed geometry also allows access to a novel ferromagnetically-coupled two-channel local moment phase. Driven by varying the interdot couplings and occurring at the point of inherent magnetic frustration, such transitions in ring structures provide a striking example of the subtle interplay between internal spin and orbital degrees of freedom in coupled quantum dot systems, and the resulting effect on Kondo physics.
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Ringer, Ashley L. "From small to big." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28089.
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Thesis (M. S.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009.Committee Chair: Sherrill, C. David; Committee Member: Bredas, Jean-Luc; Committee Member: El-Sayed, Mostafa A.; Committee Member: Harvey, Stephen C; Committee Member: Hernandez, Rigoberto.
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Gonzalez, Jose Ignacio. "Quantum Optoelectronics: Nanoscale Transport in a New Light." Diss., Available online, Georgia Institute of Technology, 2006, 2006. http://etd.gatech.edu/theses/available/etd-04062006-110542/.
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Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2006.Dr. C. P. Wong, Committee Member ; Dr. C. David Sherrill, Committee Member ; Dr. Thomas M. Orlando, Committee Member ; Dr. Mostafa A. El-Sayed, Committee Member ; Dr. Robert M. Dickson, Committee Chair.
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Liao, Rongzhen. "Quantum Chemical Cluster Modeling of Enzymatic Reactions." Doctoral thesis, Stockholms universitet, Institutionen för organisk kemi, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-43026.
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The Quantum chemical cluster approach has been shown to be quite powerful and efficient in the modeling of enzyme active sites and reaction mechanisms. In this thesis, the reaction mechanisms of several enzymes have been investigated using the hybrid density functional B3LYP. The enzymes studied include four dinuclear zinc enzymes, namely dihydroorotase, N-acyl-homoserine lactone hydrolase, RNase Z, and human renal dipeptidase, two trinuclear zinc enzymes, namely phospholipase C and nuclease P1, two tungstoenzymes, namely formaldehyde ferredoxin oxidoreductase and acetylene hydratase, aspartate α-decarboxylase, and mycolic acid cyclopropane synthase. The potential energy profiles for various mechanistic scenarios have been calculated and analyzed. The role of the metal ions as well as important active site residues has been discussed. In the cluster approach, the effects of the parts of the enzyme that are not explicitly included in the model are taken into account using implicit solvation methods. For all six zinc-dependent enzymes studied, the di-zinc bridging hydroxide has been shown to be capable of performing nucleophilic attack on the substrate. In addition, one, two, or even all three zinc ions participate in the stabilization of the negative charge in the transition states and intermediates, thereby lowering the barriers. For the two tungstoenzymes, several different mechanistic scenarios have been considered to identify the energetically most feasible one. For both enzymes, new mechanisms are proposed. Finally, the mechanism of mycolic acid cyclopropane synthase has been shown to be a direct methyl transfer to the substrate double bond, followed by proton transfer to the bicarbonate. From the studies of these enzymes, we demonstrate that density functional calculations are able to solve mechanistic problems related to enzymatic reactions, and a wealth of new insight can be obtained.
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Yuen-Zhou, Joel. "A Quantum Information Approach to Ultrafast Spectroscopy." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10317.
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In the first part of the dissertation, we develop a theoretical approach to analyze nonlinear spectroscopy experiments based on the formalism of quantum state (QST) and process tomography (QPT). In it, a quantum system is regarded as a black box which can be systematically tested in its performance, very much like an electric circuit is tested by sending a series of inputs and measuring the corresponding outputs, but in the quantum sense. We show how to collect a series of pump-probe or photon-echo experiments, and by varying polarizations and frequency components of the perturbations, reconstruct the quantum state (density matrix) of the probed system for a set of different initial conditions, hence simultaneously achieving QST and QPT. Furthermore, we establish the conditions under which a set of two-dimensional optical spectra also yield the desired results. Simulations of noisy experiments with inhomogeneous broadening show the feasibility of the protocol. A spin-off of this work is our suggestion of a witness that distinguishes between spectroscopic time-oscillations corresponding to vibronic only coherences against their electronic counterparts. We conclude by noting that the QST/QPT approach to nonlinear spectroscopy sheds light on the amount of quantum information contained in the output of an experiment, and hence, is a convenient theoretical and experimental paradigm even when the goal is not to perform a full QPT. In the second part of the thesis, we discuss a methodology to study the electronic dynamics of complex molecular systems, such as photosynthetic units, in the framework of time-dependent density functional theory (TD-DFT). By treating the electronic degrees of freedom as the system and the nuclear ones as the bath, we develop an open quantum systems (OQS) approach to TD-DFT. We formally extend the theoretical backbone of TD-DFT to OQS, and suggest a Markovian bath functional which can be readily included in electronic structure codes.
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Raber, Johan. "Quantum Chemical Studies of Chemotherapeutic Drug Cisplatin : Activation and Binding to DNA." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis Acta Universitatis Upsaliensis, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-7824.
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Dasgupta, Saswata. "Ab-Initio Implementation of Ground and Excited StateResonance Raman Spectroscopy: Application to CondensedPhase and Progress Towards Biomolecules." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1591053672243115.
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Clark, Craig R. "Sympathetic heating and cooling of trapped atomic and molecular ions." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/43757.
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Laser-cooled atomic ions have led to an unprecedented amount of control over the quantum states of matter. The Coulombic interaction allows for information to be transferred between neighboring ions, and this interaction can be used to entangle qubits for logic operations in quantum information processors. The same procedure for logic operations can be used for high resolution atomic spectroscopy, and is the basis for the most accurate atomic optical clocks to date. This thesis describes how laser-cooled atomic ions can impact physical chemistry through the development of molecular ion spectroscopy techniques and the simulation of magnetic systems by ion trap quantum computers.
A new technique developed for spectroscopy, Sympathetic Heating Spectroscopy (SHS), takes advantage of the Coulombic interaction between two trapped ions: the control ion and a spectroscopy ion. SHS uses the back action of the interrogating laser to map spectroscopy ion information onto the Doppler shift of the control ion for measurement. SHS only requires Doppler cooling of the ions and fluorescence measurement and represents a simplification of quantum logic spectroscopy. This technique is demonstrated on two individual isotopes of calcium: Ca-40(+) for cooling and Ca-44(+) as the spectroscopy ion.
Having demonstrated SHS with atomic ions, the next step was to extend the technique by loading and characterizing molecular ions. The identification of an unknown molecular ion is necessary and can be achieved by monitoring the change in motion of the two ion crystal, which is dependent on the molecular ion mass. The motion of two trapped ions is described by their normal modes, which can be accurately measured by performing resolved sideband spectroscopy of the S(1/2)-D(5/2) transition of calcium. The resolved sidebands can be used to identify unknown ions (atomic and molecular) by calculating the mass based on the observed value in axial normal mode frequencies. Again, the trapped molecular ion is sympathetically cooled via the Coulombic interaction between the Ca-40(+) and the unknown molecular ion. The sensitivity of SHS could be improved by implementing sympathetic sideband cooling and determining the heating by measuring single quanta of motion.
The ultimate limit of control would be the development of an ion trap quantum computer. Many theoretical quantum computing researchers have made bold claims of the exponential improvement a quantum computer would have over a classical computer for the simulation of physical systems such as molecules. These claims are true in principle for ideal systems, but given non-ideal components it is necessary to consider the scaling due to error correction. An estimate of the resource requirements, the total number of physical qubits and computational time, required to compute the ground state energy of a 1-D quantum Transverse Ising Model (TIM) of N spin-1/2 particles, as a function of the system size and the numerical precision, is presented. This estimate is based on analyzing the impact of fault-tolerant quantum error correction in the context of the quantum logic array architecture. The results show that a significant amount of error correction is required to implement the TIM problem due to the exponential scaling of the computational time with the desired precision of the energy. Comparison of this result to the resource requirements for a fault-tolerant implementation of Shor's quantum factoring algorithm reveals that the required logical qubit reliability is similar for both the TIM problem and the factoring problem.
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Gutierrez-Cortes, Boris Daniel. "Translationally-transformed coupled-cluster theory for periodic systems." Scholarly Commons, 2021. https://scholarlycommons.pacific.edu/uop_etds/3740.
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There are a lot of interesting problems in surface chemistry where quantum chemistry could give great insight, like reaction mechanisms in heterogeneous catalysis, the effect of surface functionalization on semiconductors, or the influence of defects on the reactivity of crystal surfaces.
Plane wave based methods applied to crystals cannot handle problems that are localized in nature like surface defects and adsorbates. On the other hand, molecular electronic structure techniques, which describe these effects and the locality of the electronic correlation well, are too computationally expensive to use on these systems.
In this work, we introduce translationally-transformed coupled-cluster (TT-CC) theory, a new electronic structure method that incorporates the periodicity of crystals and the locality of electronic correlation. This is accomplished by encoding the periodicity into the amplitudes, instead of using plane waves, in order to be able to use a local basis to reflect the decay of the electronic correlation at sufficiently large distances. This avoids the calculation of redundant amplitudes. Perfectly periodic surfaces are envisioned as reference wavefunctions for localized defects and chemical reactions.
The working equations in one dimension are derived starting from the amplitude equations of conventional coupled cluster singles and doubles (CCSD) on an infinite system and rearranging them such that the distance to an anonymous cell is an explicit degree of freedom, L. The formally infinite summations can be truncated by systematically neglecting numerically insignificant amplitudes. The generalization of the amplitude equations to higher dimensions is straightforward, albeit laborious. We show a general strategy to incorporate defects. These will be subjects of future dissertations.
We present a proof of principle for 1-dimensional chemical systems of increasing size (He, H2, Be, Ne and N2) using the 6-31G basis set. We compute the energies, with TT-CCSD, at different distances and compared them against the perfectly periodic intensive energy (PPIE) using conventional CCSD. All results, up to L=3, show that the energies of TT-CCSD converge to the PPIE. For neon, TT-CCSD shows an error of -6.2x10-6 Eh per cell against the PPIE at the bonding distance with the potential computational cost of 7 cells using CCSD, as an upper bound. For nitrogen, TT-CCSD shows an error of -2.2x10-9 Eh at 7.5 Å per cell with the same potential cost as upper bound.
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Pelmenschikov, Vladimir. "Theoretical Modeling of Enzyme Catalysis with Focus on Radical Chemistry." Doctoral thesis, Stockholm : Dept. of Physics, Stockholm university, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-513.
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Guidez, Emilie Brigitte. "Quantum mechanical origin of the plasmonic properties of noble metal nanoparticles." Diss., Kansas State University, 2014. http://hdl.handle.net/2097/17314.
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Doctor of PhilosophyDepartment of Chemistry
Christine M. Aikens
Small silver and gold clusters (less than 2 nm) display a discrete absorption spectrum characteristic of molecular systems whereas larger particles display a strong, broad absorption band in the visible. The latter feature is due to the surface plasmon resonance, which is commonly explained by the collective dipolar motion of free electrons across the particle, creating charged surface states. The evolution between molecular properties and plasmon is investigated. Time-dependent density functional theory (TDDFT) calculations are performed to study the absorption spectrum of cluster-size silver and gold nanorods. The absorption spectrum of these silver nanorods exhibits high-intensity longitudinal and transverse modes (along the long and short axis of the nanorod respectively), similar to the plasmons observed experimentally for larger nanoparticles. These plasmon modes result from a constructive addition of the dipole moments of nearly degenerate single-particle excitations. The number of single-particle transitions involved increases with increasing system size, due to the growing density of states available. Gold nanorods exhibit a broader absorption spectrum than their silver counterpart due to enhanced relativistic effects, affecting the onset of the longitudinal plasmon mode. The high-energy, high-intensity beta-peak of acenes also results from a constructive addition of single-particle transitions and I show that it can be assigned to a plasmon. I also show that the plasmon modes of both acenes and metallic nanoparticles can be described with a simple configuration interaction (CI) interpretation. The evolution between molecular absorption spectrum and plasmon is also investigated by computing the density of states of spherical thiolate-protected gold clusters using a charge-perturbed particle-in-a-sphere model. The electronic structure obtained with this model gives good qualitative agreement with DFT calculations at a fraction of the cost. The progressive increase of the density of states with particle size observed is in accordance with the appearance of a plasmon peak. The optical properties of nanoparticles can be tuned by varying their composition. Therefore, the optical behavior of the bimetallic Au[subscript](25-n)Ag[subscript]n(SH)[subscript]18[superscript]- cluster for different values of n using TDDFT is analyzed. A large blue shift of the HOMO-LUMO absorption peak is observed with increasing silver content, in accordance with experimental results.