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1
Gregory, J. "Quantum simulation of water clusters". Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599693.
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The work for this dissertation was conducted in the theoretical division of the chemistry department between October 1993 and July 1996. The thesis is concerned with computational simulation of the vibrational states of water clusters from the dimer to the hexamer. Also studied is the benzene-water dimer. To solve the nuclear Schrödinger equation for the systems in question, the diffusion Monte Carlo (DMC) method is used. This work represents an extension of DMC to larger systems than it has been applied to previously. It is shown that significant effects arise from vibrational averaging in water clusters. Good agreement is obtained with experimental results for quantities such as vibrationally averaged structures, rotational constants and tunnelling splittings. In addition, predictions are made to aid future experimental studies for the clusters discussed. The use of DMC in conjunction with more well known methods is shown to represent an extremely powerful new approach for facilitating the interaction between theory and experiment in weakly bound systems.
2
Mouhat, Félix. "Fully quantum dynamics of protonated water clusters". Thesis, Sorbonne université, 2018. http://www.theses.fr/2018SORUS056/document.
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De nos jours, il n'existe encore aucune théorie capable de proposer une description précise et quantitative du transfert de proton en solution. En effet, ce problème est complexe du fait de la grande diversité des interactions existant dans l'eau liquide, à savoir: des interactions non liantes de type Van der Waals, des liaisons faiblement covalentes et des liaisons hydrogènes remarquablement fortes. Ces dernières sont d'ailleurs à l'origine des nombreuses propriétés fascinantes de l'eau à l'échelle macroscopique. À cela s'ajoutent les effets quantiques nucléaires dus à la faible masse de l'hydrogène, qui modifient profondément la nature de la surface d'énergie potentielle décrivant le transfert de proton le long de sa coordonnée de réaction. Nous proposons dans cette thèse une approche tout quantique basée sur une description quasi exacte de la fonction d'onde du système par l'utilisation de méthodes stochastiques de type Monte Carlo Quantique. Cette technique, combinée avec le formalisme des équations de Langevin et des intégrales de chemin de Feynman, permet de simuler à un niveau de précision inédit, n'importe quel système chimique en phase gaz ou en solution. Nous appliquons cette méthodologie à des agrégats d'eau neutres ou protonés pour apporter de nouveaux éclaircissements sur les phénomènes microscopiques régissant la diffusion du proton hydraté dans de tels systèmes. Il est mis en évidence que la mobilité du proton est optimale pour des températures proches des conditions ambiantes, du fait de la compétition subtile entre les effets thermiques et quantiques nucléairesThere is no theory up to now able to provide an accurate and quantitative description of the proton transfer (PT) yet. Indeed, the complexity of the problem stems from the large diversity of the existing interactions in liquid water, namely: non bonding Van der Waals interactions, weakly covalent bonds and remarkably strong H-bonds. The latter ones are at the origin of the numerous fascinating properties of water at the macroscopic scale. In addition to such interactions, the nuclear quantum effects arising from the hydrogen light mass deeply modify the potential energy surface, and must be taken into account. In this thesis, we propose a fully quantum approach based on an almost exact description of the electronic wave function by means of Quantum Monte Carlo (QMC) methods. Our novel technique combines QMC with a Langevin-based Molecular Dynamics and the Feynman's path integral formalism. This allows one to perform fully quantum simulations of systems in gas or condensed phase, at an unprecedented level of accuracy,. We apply our approach to neutral or charged protonated water clusters to shed light on the microscopic phenomena driving the proton diffusion in such systems. We discovered that the proton hopping is optimal for temperatures close to ambient conditions, due to the subtle competition between thermal and nuclear quantum effects. This is highly suggestive of the importance of quantum nuclear effects to make PT processes - relevant for life - most efficient at room temperature
3
Schmidt, Karl. "Factorizable Module Algebras, Canonical Bases, and Clusters". Thesis, University of Oregon, 2018. http://hdl.handle.net/1794/23793.
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The present dissertation consists of four interconnected projects. In the first, we introduce and study what we call factorizable module algebras. These are $U_q(\mathfrak{g})$-module algebras $A$ which factor, potentially after localization, as the tensor product of the subalgebra $A^+$ of highest weight vectors of $A$ and a copy of the quantum coordinate algebra $\mathcal{A}_q[U]$, where $U$ is a maximal unipotent subgroup of $G$, a semisimple Lie group whose Lie algebra is $\mathfrak{g}$.
The class of factorizable module algebras is surprisingly rich, in particular including the quantum coordinate algebras $\mathcal{A}_q[Mat_{m,n}]$, $\mathcal{A}_q[G]$ and $\mathcal{A}_q[G/U]$. It is closed under the braided tensor product and, moreover, the subalgebra $A^+$ of each such $A$ is naturally a module algebra over the quantization of $\mathfrak{g}^*$, the Lie algebra of the Poisson dual group $G^*$.
The aforementioned examples of factorizable module algebras all possess dual canonical bases which behave nicely with respect to factorization $A=A^+\otimes \mathcal{A}_q[U]$. We expect the same is true for many other members of this class, including braided tensor products of such. To facilitate such a construction in tensor products, we propose an axiomatic framework of based modules which, in particular, vastly generalizes Lusztig's notion of based modules. We argue that all of the aforementioned $U_q(\mathfrak{g})$-module algebras (and many others) with their dual canonical bases are included, along with their tensor products.
One of the central objects of study emerging from our generalization of Lusztig's based modules is a new (very canonical) basis $\mathcal{B}^{\diamond n}$ in the $n$-th braided tensor power $\mathcal{A}_q[G/U]$. We argue (yet conjecturally) that $\mathcal{A}_q[G/U]^{\underline{\otimes}n}$ has a quantum cluster structure and conjecture that the expected cluster structure structure on $\mathcal{A}_q[G/U]^{\underline{\otimes}n}$ is completely controlled by the real elements of our canonical basis $\mathcal{B}^{\diamond n}$.
Finally, in order to partially explain the monoidal structures appearing above, we provide an axiomatic framework to construct examples of bialgebroids of Sweedler type. In particular, we describe a bialgebroid structure on $\mathfrak{u}_q(\mathfrak{g})\rtimes\mathbb{Q} C_2$, where $\mathfrak{u}_q(\mathfrak{g})$ is the small quantum group and $C_2$ is the cyclic group of order two.
This dissertation contains previously published co-authored material.
4
鄒鳳嬌 i Fung-kiu Chow. "Quantum statistical mechanics: a Monte Carlo study of clusters". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31224258.
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Benoit, David Michel. "Diffusion Quantum Monte Carlo simulations of hydrogen-bonded clusters". Thesis, University College London (University of London), 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.313057.
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Albring, Morten. "Towards quantum information processing with Cr3+ based heterometallic clusters". Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/towards-quantum-information-processing-with-cr3-based-heterometallic-clusters(6ff7e303-ca75-4632-986d-48bea42d96e3).html.
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An investigation of the electronic structure of some transition metal clusters comprising anti-ferromagnetically coupled, heterometallic arrays of eight metal ions that are wheel-shaped, is reported. The compounds were synthesized and provided by Dr. Grigore Timco of The University of Manchester and are formulated by their metal content as Cr7M, where M = a divalent 3d metal. Two families of wheels are the subject of this research, defined ‘green’ and ‘purple’ from their physical appearance. Within each family, the compounds are all isostructural. From simulation using a single Hamiltonian for Cr7M-purple compounds, where M = Zn, Mn, or Ni, it is shown that with only two exchange parameters, one JCr-Cr and one JCr-M, data from bulk magnetization, neutron scattering, Electron Paramagnetic Resonance (EPR) spectroscopy at multiple frequencies and specific heat measurements can be modelled and that there is transferability of parameters. Preliminary attempts to measure electron spin relaxation times for two of the purple wheels have shown values of T1 and T2 that are comparable with those of the more extensively studied green wheels and hence further studies in this area are warranted. Variable temperature Q- and W-band EPR spectra for a series of nine heterodimers comprising one green and one purple wheel, M=Zn, Mn or Ni in each case, are reported. For Cr7Zn-purple there is no magnetic exchange detected, whereas weak and quantifiable exchange is required to interpret the spectra from the other six dimers. EPR studies of three trimers of the form purple-green-purple are reported and the presence of magnetic exchange is identified by comparison with the spectra of the component single and double wheel compounds, although this is not quantified because of the numerical size of the simulations that are required. The process of comparing simulated to experimental spectra is a complex problem and one which is central to the work reported in this thesis. The problem of fitting has been investigated and two novel solutions, one based upon pixel mapping and the other based on wavelet transformation are proposed.
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Chow, Fung-kiu. "Quantum statistical mechanics a Monte Carlo study of clusters /". Hong Kong : University of Hong Kong, 2000. http://sunzi.lib.hku.hk/hkuto/record.jsp?B22424799.
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Reid, Adam. "Quantum tunnelling splittings in water clusters, from ring-polymer instanton theory". Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709028.
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Varanasi, Mohan R. "Geometries of small cadmium selenide (CdSe) clusters". Virtual Press, 2006. http://liblink.bsu.edu/uhtbin/catkey/1349770.
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The sizes, shapes, relaxed atomic positions, eigenvalues, and total energies are calculated for selected ultra-small CdSe clusters using SIESTA, a software package for electronic structure calculations and molecular dynamics simulations of molecules and solids. The properties of these bare clusters with small numbers of constituent atoms are studied using density functional theory (DFT) for energy calculations and the conjugate gradient approximation as well as simulated annealing type of molecular dynamics techniques in relaxing the structure to find the lowest energy configurations.The ab-initio norm-conserving pseudopotentials, the exchange-correlation approximation, and parameters used in the computations by Siesta software is verified using FHI98PP, a package used to generate and test the ab-initio norm-conserving pseudopotentials. The initial position of the atomic co-ordinates is determined using ancillary software written in Matlab.Department of Physics and Astronomy
10
Dai, Yafei. "Quantum and classical studies of calcium and zinc clusters and of pyrrole oligomers". Fairfax, VA : George Mason University, 2009. http://hdl.handle.net/1920/3448.
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Thesis (Ph.D.)--George Mason University, 2009.Vita: p. 131. Thesis director: Estela Blaisten-Barojas. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Title from PDF t.p. (viewed June 10, 2009). Includes bibliographical references (p. 125-130). Also issued in print.
11
Ye, Xinying. "SEMI-AUTOMATIC AND INTERACTIVE VISUALIZATION OF QUANTUM DOT NANO-STRUCTURES". University of Akron / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=akron1195496291.
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Casalenuovo, Kristen. "Density Functional Investigations of Pure and Ligated Clusters". VCU Scholars Compass, 2009. http://scholarscompass.vcu.edu/etd/1788.
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Atomic clusters are attractive candidates for building motifs for new nano-assembled materials with desirable properties. At this nano-regime of matter, the size, shape, and composition of clusters changes their electronic structure and hence their properties. Computational modeling must work hand in hand with experiment to provide robust descriptions of the geometries and energetics of atomic clusters and how they might behave in a nano-assembled material. To this end, we have investigated three distinct species as model systems: antimony oxides SbxOy (x = 1, 2; y = 0 - 3), metal ion-solvent complexes Mm(NH3)n (M = Bi, Pb; m = 1 - 2, n = 0 - 4), and quantum dots Z10H16 (Z = Si, Ge) and β-Sn12H24. Their geometries and electronic structures have been determined using gradient-corrected density functional theory. The relative stabilities for antimony oxides have been examined by the respective comparison of highest-occupied and lowest-unoccupied molecular orbital (HOMO-LUMO) gaps and atomization energies. The superior electronic stability of Sb2O3 is indicated by its closed shell structure, wide HOMO-LUMO gap calculated to be 3.11 eV, and high atomization energy of 4.21 eV. Spin-orbit corrections were necessary for accurate calculation of the metal-solvent energetics, closing the gap between experimental and theoretical values by 1.05 eV for the electron affinity of the Pb atom. Quantum dot modeling of the well-established Si and Ge as well as the less-investigated Sn illuminated the accuracy of the CEP basis sets and the B3LYP functional over other DFT computational routes for clusters containing elements beyond the third row. Throughout, the results correlate well with experiment and higher order ab initio methods where data is available. These comparisons validate the accuracy of the computational routes used. This document was prepared in the Linux Ubuntu Open Office Suite 2.4.1.
13
Skrbic, Tatjana. "Dynamics of doped [4]He and [3]He clusters from reptation quantum Monte Carlo". Doctoral thesis, SISSA, 2009. http://hdl.handle.net/20.500.11767/4779.
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In this thesis we will address these open issues in the spectroscopy of doped 4He clusters. Some of the techniques developed to this purpose will turn out to be instrumental in tackling one of the hardest problems in quantum simulation today, namely the simulation of interacting fermions. In particular, we will address the splitting of roto-vibrational lines observed in the spectra of small CO@4HeN clusters for some cluster sizes N [25], that
led in turn to ambiguous assignments of experimental lines around this size range [26]. Then, we will investigate the nature of the weak satellite band, experimentally seen to accompany the roto-vibrational line in the infrared
spectrum of CO2 solvated in 4He nanodroplets [27]. Further, the controversial convergence of CO effective rotational constant towards the nanodroplet limit will be discussed. Finally, we will study the effect that particle exchanges in a quantum solvent exhibit on the spectra of embedded molecules. This physical issue is particularly interesting because the NCRI manifests itself in quantum clusters already for extremely small system sizes. Experimentally, this effect was studied by Grebenev et al. [28, 29] by comparing the rotational spectra of OCS molecule solvated with para-hydrogen molecules, that are indistinguishable bosons, and ortho-deuterium molecules, an essentially distinguishable bosonic mixture. On the theoretical side, this issue was tackled by finite temperature simulations of N2O solvated with distinguishable particles [11]. In this thesis we will address it by zero-temperature simulations of doped 3He clusters.
The thesis is organized as follows. Quantum Monte Carlo methods are introduced in Chapter 1, with emphasis on RQMC. In Chapter 2 we present the implementation of RQMC for doped He clusters, together with several technical details of the simulation. Chapter 3 addresses the approach of CO effective rotational constant to its nanodroplet value. In Chapter 4 we cope with fine spectral features such as line splittings in small CO@4HeN clusters and the satellite band in the infrared spectra of CO2 solvated in 4He nanodroplets. To this purpose, we introduce symmetry-adapted, imaginary-time correlation functions (SAITCFs), that are specifically devised for an explicit theoretical characterization of individual excitations, as well as for an enhanced computational efficiency in the calculation of weak spectral features. Also, we show how RQMC data help to discriminate between different assignments of experimental lines proposed for CO@4HeN clusters around the cluster size where one of the splittings is observed. In Chapter 5 we address the effect of particle statistics on the rotational spectra of 3He-solvated molecules. We introduce a new method, based on a generalization of SAITCF approach that allows us to access rotational states of systems obeying Fermi statistics, still performing RQMC simulations for bosons. We apply this method to CO2
solvated with 3He atoms. Our preliminary results indicate that a certain amount of NCRI is found also in these fermionic clusters.
14
Bürgel, Christian. "Functionality of noble-metal clusters". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2009. http://dx.doi.org/10.18452/15887.
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In dieser Dissertation wurden die ungewöhnlichen und einzigartigen Eigenschaften von Edelmetall-Clustern untersucht, die durch Quantum-Confinement im Sub-Nanometer-Bereich entstehen. Dabei zeigt sich, dass die chemischen und physikalischen Eigenschaften und damit die Funktionen nicht vom Festkörper abgeleitet werden können und stark von der Anzahl der Atome abhängen. Die erzielten theoretischen Ergebnisse wurden in enger Zusammenarbeit mit experimentell arbeitenden Partnergruppen erzielt. Dabei hat sich gezeigt, dass durch die enge Kooperation zwischen Theorie und Experiment ein tiefes Verständnis von fundamentalen Prozessen und den zugrunde liegenden Mechanismen erlangt werden kann. Im Rahmen dieser Dissertation wurden die Reaktivität von geladenen Goldoxid-Clustern in der Gasphase, die ultraschnelle Dynamik von Edelmetall-Clustern und deren Komplexen sowie die optischen Eigenschaften von kleinen, deponierten Silber-Clustern untersucht und damit Beiträge geliefert, die einzigartigen Eigenschaften von Edelmetall-Clustern im Zusammenhang mit der heterogenen Katalyse und Nano-Optik besser zu verstehen.In this thesis, the unique novel properties of noble metal clusters which arise in the sub-nanometer size regime due to quantum confinement have been theoretically explored. It has been demonstrated that by adding or removing a single atom the chemical and physical properties and functionality of noble-metal clusters can strongly change. The theoretical results have been derived in close cooperation with experimental findings of partner groups demonstrating that by joint theoretical and experimental efforts thorough understanding of fundamental processes and underlying mechanisms can be achieved. This thesis addresses the reactivity of charged gas-phase gold-oxide clusters in the context of the heterogeneous gold nano-catalysis, the ultrafast dynamical properties of noble-metal clusters and their complexes, and the optical properties of silver clusters at surfaces.
15
Pichugin, Kostyantyn. "ELECTRONIC STRUCTURE AND REACTION DYNAMICS OF MOLECULAR AND CLUSTER ANIONS VIA PHOTOELECTRON IMAGING". Diss., The University of Arizona, 2010. http://hdl.handle.net/10150/194339.
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The electronic structure and reaction dynamics of molecular and cluster anions in the gas phase has been investigated using negative ion velocity-map imaging photoelectron spectrometer. Photoelectron images provide important information about both energies and symmetries of the parent anion orbitals from which photoelectron originates. The symmetry and the ordering of several low-lying electronic states of neutral nitromethane (X¹A′, a³A″, b³A″, and A¹A″) are assigned based on a group theoretical analysis of the transitions angular distributions and the results of DFT calculations. The through-bond electronic coherence in meta- and para-dinitrobenzene anions is explored by recording a series of photoelectron images in 532-266 nm wavelength range. Photoelectron angular distributions for both isomers exhibit oscillatory behavior characteristic of the quantum interference effect, suggesting that dinitrobenzene anions retain their high symmetry electronic structures in the gas phase. Photoelectron imaging experiments on [O(N₂O)(n)]⁻, n =0–9 at 266 and 355 nm provide clear evidence of a switch from the cova)lent NNO₂⁻ cluster core to the atomic O⁻ core occurring between n = 3 and 4. The experimental results and theoretical modeling indicate that despite the greater stability of NNO₂⁻ relative to the O⁻ + N₂O⁻ dissociation limit, an O⁻ cluster core becomes energetically favored over NNO₂⁻ for n > 3, due to the more efficient solvation of the atomic anion. The photodissociation dynamics of I₂⁻ and IBr⁻ anions on the respective A' excited-state anion potentials is effectively unraveled in 780 nm pump - 390 nm probe time-resolve experiments. The time-dependent photoelectron spectra and classical trajectory calculations of the IBr⁻ dissociation provide the first rigorous dynamical test of the recently calculated A′ potential for this system. The photoelectron anisotropy cyclic variation observed in photodissociation of I₂⁻ is interpreted in the context of dual-center quantum interference model. The 390 nm pump – 390 nm probe experimental data reveal fast (≤100 fs) and delayed (~ 700 fs) appearance of the I⁻ channel in the photodissociation of I₂Cl⁻ and BrICl⁻ anions respectively. The difference in the reaction time-scales is attributed to the distinct dissociation pathways available for the anions to form I⁻ product.
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Wu, Lu. "STRUCTURES AND ELECTRONIC STATES OF SMALL GROUP 3 METAL CLUSTERS". UKnowledge, 2014. http://uknowledge.uky.edu/chemistry_etds/32.
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Group 3 metal clusters are synthesized by laser vaporization in a pulsed cluster beam source and identified with laser ionization time-of-flight mass spectrometry. The adiabatic ionization energies and vibrational frequencies of these clusters are measured using mass-analyzed threshold ionization (MATI) spectroscopy. Their structures and electronic states are determined by combining the MATI spectra with quantum chemical calculations and spectral simulations. This dissertation focuses on the study of several small molecules, which include LaO2, La2, M2O2, M3O4, M3C2, and La3C2O, where M = Sc, Y, and La. Except for La2, these molecules exhibit strong ionic characters between the metal and oxygen or carbon atoms and can be described as [O-][La2+][O-], [M2+]2[O2-]2, [M8/3+]3[O2-]4, [M2+]3[C3-]2, and [La8/3+]3[C3-]2[O2-]. The interactions between the metal atoms form covalent bonds, which can be described by a triple bond in La2, a two-center two electron bond in M2O2, a three-center one electron bond in M3O4, and a three-center three electron bond in M3C2. In addition, the electron in the non-bonding highest occupied molecular orbital (HOMO) is localized in the La 6s orbital in LaO2 and La3C2O. The ground states of these molecules are all in low electron-spin states with the spin multiplicities of 1 or 2. Although the ground electronic state of LaO2 is a linear structure, the excited quartet state of the molecule is determined to be a bent structure. M2O2 and M3O4 have the planar rhombic and cage-like structures, respectively; whereas M3C2 has a trigonal bipyramid structure. La3C2O is formed by oxygen binding with two La atoms of La3C2. Ionization removes a metal-based (n+1)s electron in all neutral molecules, and the resultant ions have similar geometries to those of the corresponding neutral states. In the case of La2, additional ionization of a La 5d electron is also observed.
17
Wang, Ruizhe. "Magnetic fluctuations and clusters in the itinerant ferromagnet Ni-V close to a disordered quantum critical point". Kent State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=kent1555936445433091.
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Averkiev, Boris. "Geometry and Electronic Structure of Doped Clusters via the Coalescence Kick Method". DigitalCommons@USU, 2009. http://digitalcommons.usu.edu/etd/408.
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Developing chemical bonding models in clusters is one of the most challenging tasks of modern theoretical chemistry. There are two reasons for this. The first one is that clusters are relatively new objects in chemistry and have been extensively studied since the middle of the 20th century. The second reason is that clusters require high-level quantum-chemical calculations; while for many classical molecules their geometry and properties can be reasonably predicted by simpler methods.
The aim of this dissertation was to study doped clusters and explain their chemical bonding. The research was focused on three classes of compounds: aluminum clusters doped with one nitrogen atom, planar compounds with hypercoordinate central atom, partially mixed carbon-boron clusters, and transition metal clusters. The geometry of the two latter classes of compounds was explained using the concept of aromaticity, previously developed in our group.
Also the Coalescence Kick Method for finding global minima structure and low-lying isomers was implemented, tested, and applied to the considered cluster systems. Tests showed that the Kick Method works faster than other methods and provides reliable results. It finds global minima even for such large clusters as B17- and B19- in reasonable time.
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North, Stephen Michael. "Electronic structure of GaSb/GaAs and Si/Ge quantum dots". Thesis, University of Newcastle Upon Tyne, 2001. http://hdl.handle.net/10443/551.
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There are significant differences between experiment and theoretical calculations of the electronic structure of GaSb/GaAs self-assembled quantum dots. Using a multi-band effective mass approximation it is shown that the influence of size and geometry of quantum dots has little or no effect in determining the hydrostatic strain. Furthermore, the valenceband ground state energies of the quantum dots studied are surprisingly consistent. This apparent paradox attributed to the influence of biaxial strain in shaping the heavy-hole and light-hole potentials. Consequently, it is shown that a simple, hydrostatically derived potential is insufficient to accurately describe the electronic structure of such quantum dots. In addition, using the latest experimental results measuring the conductionband offset, it has been shown that much better experimental contact may be achieved for the magnitude of the transition energies derived compared to theoretically derived transition energies. The transition energies of Si/Ge self-assembled quantum dots has also been calculated. In particular, a range of quantum dot structures have been proposed that are predicted to have an optical response in the 3-5 micron range.
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Bhowmick, Somnath. "Theory on lower bound energy and quantum chemical study of the interaction between lithium clusters and fluorine/fluoride". Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4394.
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En chimie quantique, le principe variationnel est largement utilisé pour calculer la limite supérieure de l'énergie exacte d'un système atomique ou moléculaire. Des méthodes pour calculer la valeur limite inférieure de l'énergie existent mais sont bien moins connues. Une méthode précise pour calculer une telle limite inférieure permettrait de fournir une barre d'erreur théorique pour toute méthode de chimie quantique. Nous avons appliqué des méthodes de type variance pour calculer différentes énergies limites inférieures de l'atome d'hydrogène en utilisant des fonctions de base gaussiennes. L'énergie limite supérieure se trouve être toujours plus précise que ces différentes limites inférieures, i.e. plus proche de l'énergie exacte. L'importance de points singuliers sur l'évaluation de valeurs moyennes d'opérateurs quantiques a également été soulignée.Nous avons étudié les réactions d'adsorption d'un atome de fluor et d'un ion fluorure sur de petits agrégats de lithium Li$_n$ (n=2-15), à l'aide de méthodes de chimie quantique précises. Pour le plus petit système, nous avons montré que la formation de complexes stables Li$_2$F et Li$_2$F$^-$ se produit par un transfert d'électrons sans barrière et à longue portée, de Li$_2$ vers F pour le système neutre et l'inverse pour le système anionique. De telles réactions pourraient être rapides à très basse température. De plus, les complexes formés présentent des caractéristiques uniques de "longue liaison". Pour les systèmes plus gros Li$_n$F/Li$_n$F$^-$ ($n\geqslant4$), nous avons montré que les énergies d'adsorption peuvent être aussi grandes que 6~eV selon le site d'adsorption et que plus d'un état électronique est impliqué dans le processus d'adsorption. Les complexes formés présentent des propriétés intéressantes de "super alcalins" et pourraient servir d'unités de base dans la synthèse de composés à transfert de charge avec des propriétés ajustablesIn quantum chemistry, the variational principle is widely used to calculate an upper bound to the true energy of an atomic or molecular system. Methods for calculating the lower bound value to the energy exist but are much less known. An accurate method to calculate such a lower bound would allow to provide a theoretical error bar for any quantum chemistry method. We have applied variance-like methods to calculate different lower bound energies of a hydrogen atom using Gaussian basis functions. The upper bound energy is found to be always more accurate than the lower bound energies, i.e. closer to the exact energy. The importance of singular points on mean value evaluation of quantum operators has also been brought to attention.The adsorption reactions of atomic fluorine (F) and fluoride (F$^-$) on small lithium clusters Li$_n$ (n=2-15) have been investigated using accurate quantum chemistry ab initio methods. For the smallest system, we have shown that the formation of the stable Li$_2$F and Li$_2$F$^-$ complexes proceeds via a barrierless long-range electron transfer, from the Li$_2$ to F for the neutral and conversely from F$^-$ to Li$_2$ for the anionic system. Such reactions could be fast at very low temperature. Furthermore, the formed complexes show unique long bond characteristics. For the bigger Li$_n$F/Li$_n$F$^-$ systems ($n\geqslant 4$), we have shown that the adsorption energies can be as large as 6~eV depending on the adsorption site and that more than one electronic state is implied in the adsorption process. The formed complexes show interesting "superalkali" properties and could serve as building blocks in the synthesis of charge-transfer compounds with tunable properties
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Paolini, Stefano. "Rotational dynamics of molecular impurities solvated in 4He clusters. A computational study based on reptation quantum Monte Carlo". Doctoral thesis, SISSA, 2006. http://hdl.handle.net/20.500.11767/4042.
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The thesis is organized as follows: in the first chapter we provide a short description
of the scenario which frames our work. Some significant experiments
are presented, together with the questions raised by them, the theoretical investigations
they stimulated, and the open issues. The second chapter describes the
reptation quantum Monte Carlo method and its theoretical foundations. Technical
aspects (the choice of the trial functions, the procedure to calculate the cluster
rotational energies) are discussed in third chapter. We also report some studies
on the reptation algorithm. In the fourth chapter we apply RQMC for the interpretation
of the infrared spectra of CO@HeN [5]. The fifth chapter addresses the
problem of the evolution of the rotational dinamics of He solvated rotors toward
the nanodroplet regime; two paradigmatic cases, OCS@HeN and HCN@HeN , are
studied. Our conclusions are drawn in the last chapter.
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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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El-Khawaldeh, Amir Verfasser], Hans-Jörg [Akademischer Betreuer] [Kull i Dieter [Akademischer Betreuer] Bauer. "Quantum Vlasov theory of Mie oscillations in metal clusters : a self-consistent approach to quantum surface effects in nanoparticles / Amir El-Khawaldeh ; Hans-Jörg Kull, Dieter Bauer". Aachen : Universitätsbibliothek der RWTH Aachen, 2018. http://d-nb.info/1171818513/34.
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El-Khawaldeh, Amir [Verfasser], Hans-Jörg [Akademischer Betreuer] Kull i Dieter [Akademischer Betreuer] Bauer. "Quantum Vlasov theory of Mie oscillations in metal clusters : a self-consistent approach to quantum surface effects in nanoparticles / Amir El-Khawaldeh ; Hans-Jörg Kull, Dieter Bauer". Aachen : Universitätsbibliothek der RWTH Aachen, 2018. http://d-nb.info/1171818513/34.
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Mello, Victor Giovanni Pina de. "Cálculo da energia de correlação de pequenos clusters de lítio via teoria da informação". Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/4940.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this dissertation, we use the information theory to obtain the correlation energy of small lithium clusters. Unlike conventional methods for calculation of the correlation energy, the method used here requires only a single calculation in a Hartree-Fock level. With this calculation we obtain a total information energy of the clusters, through the addition of information energy of each atom of the cluster. The correlation energy is a bilinear function of the information energy and number of electrons in the cluster. In order to obtain a general equation for the correlation energy, we use the correlation energy expression that has been obtained through the information theory to t the exact energy of the system using a Monte Carlo quantum simulation of clusters ranging from 1 to 8 lithium atoms. The e ciency of the general equation for the correlation energy has been veri ed through the calculation of the binding energy for clusters of up to 9 and 10 atoms which have not been used in the obtained t. The obtained values for the binding energies are in good agreement with the experimental measurements using photoelectron spectroscopy.
Nesta dissertação obtemos a energia de correlação de pequenos clusters formados por átomos de Lí tio via teoria da informa ção. Diferentemente dos m étodos usuais para o c álculo da energia de correlação, o m étodo aqui utilizado requer somente um c álculo no n ível Hartree-Fock. Deste c álculo se obt em a energia de informação do cluster atráv es da soma das energias de informação dos átomos individuais do cluster. A energia de correlação e uma função bilinear da energia de informa ção e do n úmero de el étrons no cluster. A m de obter uma expressão geral para a energia de correla ção dos clusters de L tio, n os ajustamos a expressão obtida com seus respectivos coe cientes a energia exata deste sistema, calculada usando a simula ção de Monte Carlo quântico para clusters variando de 1 a 8 atomos. A veri cação dessa expressão para a energia de correla ção foi avaliada para clusters maiores não usados no ajuste atrav es do c álculo da energia de liga ção (para cluster de 9 e 10 átomos), os resultados indicam valores em bom acordo com as medidas experimentais usando espectroscopia de fotoel etrons.
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Besse, Larissa Zibordi. "Estudo da adsorção de hidroxila, água e etanol sobre clusters de metais de transição com 13 átomos". Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/75/75134/tde-18082015-103454/.
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A escolha do catalisador no processo de reforma a vapor de etanol é de suma importância na determinação da eficiência de produção de hidrogênio, utilizado em células combustíveis. Superfícies de metais de transição (MT) são normalmente utilizadas como catalisadores, porém resultados experimentais têm apresentado que clusters MT suportados em óxidos são mais eficientes e mais seletivos que seus análogos macroscópicos. Assim, para se compreender o mecanismo de interação de etanol e água, e também do radical hidroxila (simples caracterização da ligação O-H), com clusters mágicos MT13 (MT = Ni, Cu, Pd, Ag, Pt e Au), foram utilizados cálculos ab initio baseados na teoria do funcional da densidade com o funcional proposto por Perdew-Burke-Ernzerhof (PBE) e a aproximação de Tkatchenko-Scheffler para as correções de van der Waals (vdW). Os orbitais de Kohn-Sham foram expandidos através de orbitais numéricos centrados nos átomos (NAOs), que estão implementados no pacote Fritz Haber Institute - ab initio molecular simulations (FHI-aims). Foi encontrado que clusters 3d e 4d apresentam configurações mais compactas, enquanto os sistemas de Pt13 e Au13 adotam geometrias mais abertas. Em particular, Au13 possui uma estrutura planar como configuração mais estável, enquanto o seu primeiro isômero mais energético possui uma estrutura 3D. Para os cálculos de adsorção foram selecionados os clusters de menor energia (LOW), e os clusters icosaédricos (ICO). Observou-se que a hidroxila (OH) não possui sítio preferencial ou dependência geométrica durante adsorção, enquanto água e etanol se ligam preferencialmente através do oxigênio no sítio top. Para o caso do etanol também é possível se encontrar isômeros com energias mais altas, na ordem de kT, que se ligam via hidrogênio do grupo CH e que são prováveis a temperatura ambiente. OH possui um elétron desemparelhado, o que favorece a formação de ligações químicas entre a molécula e MT13, de forma que a energia de adsorção varia entre -4.11 e -2.94 eV, condizente com a transferência de carga do cluster para a molécula. Quando se compara a adsorção de água (etanol) sobre superfícies MT(111) e MT13, nota-se que a energia de adsorção, que segue a ordem 3d > 4d > 5d para MT(111), é maior em relação aos clusters e varia entre -255 meV (-317 meV) < -Ead < -670 meV (-837 meV); porém a magnitude da interação ocorre na ordem 3d > 4d, mas 4d < 5d. A quebra da tendência pode ser explicada através dos efeitos de tamanho, que potencializam a atividade catalítica dos clusters de platina e ouro. É possível se observar ainda que as moléculas interagem mais fortemente com os MT13 dos metais com orbitais d semi-preenchidos, uma vez que esses proporcionam um maior rearranjo da densidade eletrônica. Ocorre uma pequena transferência de carga das moléculas de água e etanol para os clusters, apontando para a fraca interação. Cálculos vibracionais suportam as modificações estruturais e energéticas que ocorrem no sistema, onde νO-H aumenta para a hidroxila e diminui para água e etanol.The choice of the catalyst in steam reforming of ethanol process has a central hole in determining the hydrogen production efficiency, used in fuel cells. Transition-metal (TM) surfaces are commonly used as catalysts, but experimental results have shown that TM clusters supported on oxides are more efficient and more selective than their macroscopic analogues. Therefore, to understand the mechanism of interaction between ethanol and water, and also the hydroxyl radical (simple characterization for the O-H bonding) with TM13 magic clusters (TM = Ni, Cu, Pd, Ag, Pt and Au), we performed ab initio calculations based on density functional theory within the functional proposed by Perdew-Burke-Ernzerhof (PBE) and Tkatchenko-Scheffler approach for van der Waals corrections (vdW). The Kohn-Sham orbitals were expanded by numerical atom-centered orbitals (NAOs), which are implemented at the Fritz Haber Institute - ab initio molecular simulations package (FHI-AIMS). We found that clusters 3d and 4d prefer more compact configurations, while Pt13 and Au13 systems adopt more open geometries. In particular case of Au13, our lowest energy configuration is a planar structure, while the first high energy isomer is a 3D configuration. For adsorption calculations, we selected the lowest energy configurations (LOW), and the icosahedral clusters (ICO). We observed that the hydroxyl (OH) has no adsorption site preference and also it does not show a geometric trend, while water and ethanol preferentially bind via oxygen at the top site. In the case of ethanol, it is also possible to find isomers with higher energies, which bind via hydrogen from CH group, and they are probable at room temperature. OH has an electron unpaired, which favors the formation of chemical bonds between the molecule and TM13, so that the adsorption energy varies from -4.11 to -2.94 eV, consistent with charge transfer from cluster to the molecule. When we compare the water and ethanol adsorption on surfaces TM(111) and on TM13, we note that the adsorption energy, that follows the order 3d > 4d > 5d for MT(111), is higher compared to the clusters energy. For water (ethanol), this property varies between -255 meV (-317 meV) < -Ead < -670 meV (-837 meV); however, the magnitude of the interaction occurs in the order 3d > 4d, but also 4d < 5d. The breaking of the trend can be explained by size effects, that enhances the catalytic activity of platinum and gold clusters. It is also observed that the stronger interaction between molecules and TM13 with d-orbitais partly occupied, since these metals provide a higher rearrangement of the electron density. There is a small charge transference from water and ethanol molecules to the clusters, pointing to the weak interaction. Vibrational calculations support the structural and energetic changes that occur in the system, where νO-H increases for the hydroxyl and decreases to water and ethanol.
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Oswald, Sönke. "Weak Hydrogen Bonds to Molecular Nitrogen and Oxygen as Experimental Benchmarks for Quantum Chemistry". Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2019. http://hdl.handle.net/11858/00-1735-0000-002E-E5E0-3.
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Zhang, Qi. "The fabrication and characterisation of quantum dots, wires and wire net works". Thesis, De Montfort University, 1996. http://hdl.handle.net/2086/4182.
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Chaves, Anderson Silva. "Estudo teórico das propriedades estruturais, eletrônicas e reatividade de clusters de metais de transição". Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/76/76132/tde-24042015-083608/.
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Clusters sub-nanométricos de metais de transição (TM) têm atraído grande atenção devido às suas propriedades físicas e químicas singulares, muito diferentes dos homólogos na fase bulk. Estas propriedades incomuns podem variar de acordo com o tamanho, a composição e o estado de carga para clusters em fase gasosa. Uma compreensão aprofundada da evolução das propriedades em função de tais parâmetros é um pré-requisito necessário para potencializar diversas aplicações, desde catálise até armazenamento magnético, bem como para responder questões fundamentais relacionadas com a estabilidade intrínseca desses sistemas. Porém, esse entendimento ainda não é satisfatório. Neste projeto, usando cálculos de primeiros princípios baseados na teoria do funcional da densidade dentro da aproximação de gradiente generalizado na formulação proposta por Perdew-Burke-Ernzerhoff, investigamos as estruturas atômicas, as propriedades eletrônicas e a estabilidade de todos os TM (30 elementos) clusters unários em função do tamanho (2 – 15 átomos). Para estudar a influência da carga, consideramos clusters de Cun e Ptn (n = 2 – 14) nos estados catiônico, neutro e aniônico, enquanto os efeitos de composição foram considerados para clusters bimetálicos de PtnCum com m = 2,3, · · · ,14 e n = 0,1, · · · ;,m. Nossos resultados sugerem que: (i) Os mecanismos de estabilização para os TM clusters unários baseiam-se na natureza das ligações químicas, em que a ocupação dos orbitais d ligantes ou antiligantes e a hibridização s – d afetam fortemente as propriedades. Por exemplo, a maioria dos TM clusters mágicos são acompanhados por picos de hibridização s – d e momentos de dipolo elétrico nulos.(ii) Diferentes parâmetros afetam as estruturas de TM clusters nesse regime de tamanho, tais como, propriedades do átomo livre, interações magnéticas de troca e efeitos relativísticos. (iii) As propriedades são muito susceptíveis ao estado de carga; em particular, as estruturas tendem a diminuir a coordenação atômica quando um elétron é adicionado ao sistema, o que afeta fortemente a transição 2D-3D. (iv) Energia de excesso negativa foi obtida para a maioria dos Pt–Cu clusters, o que fornece uma forte evidência para a formação de clusters bimetálicos. Além disso, nossas análises sugerem que a formação de estruturas tipo caroço(Cu)-casca(Pt) começa neste regime de tamanho, afim de liberar energia de tensão. (v) O centro de gravidade dos estados d ocupados da liga Pt–Cu varia com a composição, e se aproxima do orbital mais alto ocupado para composições em torno de 50%-50%. Em particular, nossos cálculos sugerem um aumento na energia de adsorção de CO e NO sobre Pt7Cu6 em comparação com os clusters unários de Pt13 e Cu13. Consequentemente, estes resultados constituem uma base para compreender a formação de partículas maiores bem como para investigar sistemas mais complexos e realistas, como reações químicas de sistemas moleculares adsorvidos sobre TM clusters estabilizados por ligantes ou suportados.Sub-nanometre sized transition metal (TM) clusters have attracted great attention due to their unique physical and chemical properties, very different from the bulk counterparts. These unusual properties can vary with size, composition and state of charge for gas-phase clusters. An in-depth understanding of the properties evolution in function of such parameters is a necessary prerequisite to leverage diverse applications, from catalysis to magnetic storage, as well as to answer fundamental questions related with the intrinsic stability of these systems. However, this understanding is not yet satisfactory. In this project, using first-principles calculations based on density functional theory within the generalized gradient approximation in the formulation proposed by Perdew-Burke-Ernzerhoff, we investigate the atomic structures, electronic properties and stability of all TM (30 elements) unary clusters in function of size (2 – 15 atoms). To study the influence of the charge we considered Cun and Ptn (n = 2 – 14) clusters in the cationic, neutral and anionic states, while compositional effects were considered for bimetallic PtnCum–n clusters with m = 2,3, · · · ,14 and n = 0,1, · · · ,m. Our results suggest that: (i) The stabilization mechanisms for unary clusters are based on the nature of chemical bondings, on which the occupation of the bonding or antibonding d orbitals and the s – d hybridization strongly affect the properties. For example, most magic TM clusters are accompanied by peaks in s – d hybridization and null electric dipole moments.(ii) Different parameters affect TM cluster structures in that size regime, such as, free-atom properties, magnetic exchange interactions and relativistic effects. (iii) The properties are very susceptible to the charge state; in particular, the structures tend to decrease the atomic coordination when one electron is added to the system, which strongly affects the 2D-3D transition. (iv) Negative excess energy was obtained for the most PtCu clusters, which provides a strong evidence for the formation of these bimetallic clusters. Moreover, our analyzes suggest that the formation of core(Cu)-shell(Pt) like structures starts at this size regime, in order to release strain energy. (v) The center of gravity of the occupied d states of the Pt–Cu alloy vary with composition and approaches to the highest occupied molecular orbital for compositions around 50%-50%. In particular, our calculations suggest an increase in the adsorption energy of CO and NO on Pt7Cu6 in comparison with Pt13 and Cu13 unary clusters. Thus, these results form a basis to understand the formation of greater particles as well as to investigate more complex and realistic systems, such as chemical reactions of molecular systems adsorbed on ligand stabilized or supported TM clusters.
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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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Moreira, Emanuel Melo Isaac. "O efeito da correlação eletrônica em clusters de Boro aromáticos: um estudo via Monte Carlo quântico". Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/5096.
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Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq
In this work we used a combination of density functional theory and quantum Monte Carlo methods to study the effect of electron correlation on stability and aromaticity of anionic Boron clusters (B− 3 and B− 4 ). We found that, in general, the cyclic isomer is energetically more stable than its open linear counterpart. Based on principles of minimum energy and electrophilicity, and maximum hardness, diffusion Monte Carlo indicates that B− 3 cluster is aromatic, however, the results are not conclusive with respect to the B− 4 cluster. Calculations were also performed within the Hartree–Fock approximation. From the obtained results, we analysed the impact of the electron correlation effects in these clusters and found that the correlation of the electrons contributes significantly to the ionization potential and electron affinities varying between 31% and 66% of their total values.
Neste trabalho combinamos a teoria do funcional da densidade com o método de Monte Carlo quântico para estudar o efeito da correlação eletrônica na estabilidade e na aromaticidade de clusters de boro aniônicos (B− 3 e B− 4 ). Verificamos que, em geral, o isômero cíclico é energeticamente mais estável que sua respectiva estrutura linear. Com base no princípio da mínima energia e eletrofilicidade, e máxima dureza, o método Monte Carlo por difusão indicou que o cluster B− 3 é aromático, porém, os resultados para o cluster B− 4 ainda não são conclusivos. Realizamos também cálculos com a aproximação de Hartree-Fock. Dos resultados obtidos, analisamos o impacto do efeito da correlação nesses clusters e verificamos que a correlação dos elétrons contribuem significativamente para o potencial
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Gebretsadik, Adane Samuel Gebretsadik. "INTERPLAY BETWEEN CHEMICAL AND MAGNETIC DISORDER IN SELECTED ALLOYS CLOSE TO A FERROMAGNETIC QUANTUM PHASE TRANSITION". Kent State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1527609152939816.
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Damasceno, Junior Jose Higino. "Propriedades eletrônicas e estruturais de clusters metálicos via métodos ab initio". Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/4846.
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Fundação de Amparo à Pesquisa do Estado de Goiás - FAPEG
Clusters systems are very different from molecules or their bulk materials, since they exhibit many specific properties. As example, the bond in metallic clusters of metallic atoms is intermediate between metallic and covalent bonding. In general, the structural and electronic properties of these systems are very difficult to measure experimentally, and therefore theoretical modeling is very important in characterizing them. In this thesis, we employed ab initio methods to study metallic clusters such as the aluminum hydride clusters as well as a few aromatic metal clusters. The optimized geometries of the studied clusters have been determined using DFT. The electronic structures of these systems were investigated using the QMC methods. The calculations were carried out within the Variational (VMC) and fixed-node diffusion (DMC) quantum Monte Carlo methods. The calculations are also performed in the Hartree-Fock (HF) approximation in order to analyze the impact of electron correlation. With regards the aluminum hydride clusters, the total atomic binding energy impact varies from ~20% up to about ~50%, whereas for the electron binding energy it ranges from ~1% up to ~73%. The decomposition of the electron binding energies clearly shows that both charge redistribution and electron correlation are important in determining the detachment energies, whereas electrostatic and exchange interactions are responsible for the ionization potential. For the aromatic metal clusters, the presence of a dopant plays important role in their electronic properties enhancing their binding energy, electron affinity, hardness and resonance energy.
Clusters são sistemas bastante diferentes de moléculas e sólidos, pois exibem propriedades bastante peculiares. Por exemplo, a ligação em um cluster metálico tem intensidade intermediária entre as ligações covalentes e metálicas. Em geral, as propriedades eletrônicas e estruturais desses sistemas são bastante difíceis de serem medidas experimentalmente e, portanto, uma modelagem teórica é muito importante na caracterização desses. Nesta Tese, utilizamos métodos ab initio para estudar clusters metálicos, tal como clusters de hidretos de alumínio assim como também alguns clusters metálicos aromáticos. As estruturas geométricas dos clusters estudados foram otimizadas via DFT. A estrutura eletrônica desses clusters foi investigada usando o método de Monte Carlo Quântico Variacional (MCQD) e de difusão (MCQD) com aproximação de nós fixos. Os cálculos também foram realizados a partir da aproximação de Hartree-Fock, afim de se analisar o impacto da energia de correlação eletrônica. Para os hidretos de alumínio, a energia de correlação eletrônica tem impacto na energia total de ligação variando de 20% a 50%. Da mesma maneira, a energia de ligação de um elétron ao cluster tem grande contribuição da energia de correlação eletrônica, variando de 1% a 73%. A decomposição da energia de ligação mostra claramente que a relaxação e a correlação eletrônica são importantes na determinação da afinidade eletrônica, enquanto que a interação de troca eletrostática é responsável pelo potencial de ionização. Para os clusters aromáticos, a presença do dopante desempenha um importante papel nas propriedades desses clusters, uma vez que otimiza a energia de ligação, a afinidade eletrônica, a dureza e a energia de ressonância.
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Qi, Luo [Verfasser], Maria [Akademischer Betreuer] Chekhova, Maria [Gutachter] Chekhova i Joachim von [Gutachter] Zanthier. "Non-classical multi-photon light from clusters of colloidal quantum dots / Luo Qi ; Gutachter: Maria Chekhova, Joachim von Zanthier ; Betreuer: Maria Chekhova". Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2019. http://d-nb.info/1180028422/34.
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Prabha, FNU Sweta. "Theoretical Studies of the Structure and Stability of Metal Chalcogenide CrnTem (1≤n≤6, 1≤m≤8) clusters". VCU Scholars Compass, 2019. https://scholarscompass.vcu.edu/etd/5774.
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In the presented work, first principle studies on electronic structure, stability, and magnetic properties of metal chalcogenide, CrnTem clusters have been carried out within a density functional framework using generalized gradient functions to incorporate the exchange and correlation effects. The energetic and electronic stability was investigated, and it was found that they are not always correlated as seen in the cluster Cr6Te8 which has smaller gap between its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) and a high electron affinity of 3.39 eV indicating lower electronic stability whereas higher fragmentation energy indicating energetic stability. The high electron affinity shows that the stability of Cr6Te8 cluster can be enhanced by adding charge donating ligands including PEt3 to form stable Cr6Te8(PEt3)6 clusters as seen in experiments. The other cluster of interest was Cr4Te6 in which energetic stability was accompanied with electronic inertness marked by its large HOMO-LUMO gap, non-magnetic ground state and high fragmentation energy.
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Jantke, Laura-Alice [Verfasser]. "From Zintl Phases to Novel Tetrel Allotropes - Quantum Chemical Considerations of Zintl Phases and Zintl Clusters lead to Novel Tetrel Modifications / Laura-Alice Jantke". München : Verlag Dr. Hut, 2018. http://d-nb.info/1149579161/34.
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Xu, Liang. "Simulating electronic-structure properties of atomic clusters by Ab-initio calculations, and inter-nuclear quantum-statistical effects of molecules from an integration-free path-integral method /Xu Liang". HKBU Institutional Repository, 2016. https://repository.hkbu.edu.hk/etd_oa/344.
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In this dissertation, we have employed some well-established electronic-structure methods [e.g., density functional theory (DFT) and time-dependent DFT (TD-DFT)] to investigate the potential energy surfaces for 2s 2p excitation of beryllium atomic clusters, attempting to provide direct computational support for the mechanism of a newly invented laser spectroscopy. The computing time of single-point energy calculations for a series of beryllium clusters from using TD-DFT has been compared with that from a higher-level coupled-cluster method, in order to demonstrate the computational practicality of TD-DFT methods. Meanwhile, to benchmark the accuracy of TD-DFT methods, the state properties such as the equilibrium inter-atomic distance and dissociation energy of beryllium clusters calculated by us are compared with experimental results and other computational values where available. Furthermore, we have defined the fork intersections to characterize the position where the excited states can be treated as degenerate = Moreover, to shed some light on the reaction mechanism of a Diels-Alder reaction between isoprene and maleic anhydride, we have investigated the kinetic isotope effects (KIE) of the reaction. To further include inter-nuclear quantum-statistical effects (i.e., the quantum tunneling effect and anharmonicity), an automated integration-free path-integral (AIF-PI) method developed by our group in recent years based on Kleinert's variational perturbation theory has been used. The KIE values produced by the AIF-PI method can be used to clearly distinguish between the two isomeric transition-state structures, and determine the actual rate-limiting transition state. By virtue of the AIF-PI method, we have also analyzed the quantum tunneling effects and anharmonicity separately, which are excluded in conventional Bigeleisen equation. Furthermore, the influence of different numbers of quantized nuclei on the KIE values using base-catalyzed RNA 2'-O-transphosphorylation models as examples are explored, by systematically increasing the number of quantized nuclei from 1 to 16 (fully quantized)
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Poblotzki, Anja. "Intermolecular energy scales based on aromatic ethers and alcohols". Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2019. http://hdl.handle.net/21.11130/00-1735-0000-0003-C1AD-0.
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Rupel, Dylan, i Dylan Rupel. "Quantum Cluster Characters". Thesis, University of Oregon, 2012. http://hdl.handle.net/1794/12400.
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We de ne the quantum cluster character assigning an element of a quantum torus to each
representation of a valued quiver (Q; d) and investigate its relationship to external and internal
mutations of a quantum cluster algebra associated to (Q; d). We will see that the external mutations
are related to re
ection functors and internal mutations are related to tilting theory. Our
main result will show the quantum cluster character gives a cluster monomial in this quantum
cluster algebra whenever the representation is rigid, moreover we will see that each non-initial
cluster variable can be obtained in this way from the quantum cluster character.
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López, José G. "Theoretical studies of the dynamics and spectroscopy of weakly bound systems". The Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=osu1127220592.
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Nicolas, David. "étude d'agrégats d'oxydes de terres rares". Phd thesis, Université Claude Bernard - Lyon I, 2007. http://tel.archives-ouvertes.fr/tel-00170447.
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Ce travail concerne la synthèse et l'étude de nanoparticules d'oxydes de terres rares. Les agrégats sont synthétisés par LECBD (Low Energy Cluster Beam Deposition) puis déposés sur différents substrats pour analyse. La spectroscopie de photoélectrons X montre que les agrégats produits possèdent la stoechiométrie du matériau massif. Les dépôts sont imagés par microscopie électronique. Dans les conditions usuelles les agrégats déposés ont une taille faiblement dispersée autour d'une valeur moyenne de 3 nm. Les agrégats sont alors cristallisés et présentent une forme particulière, le dodécaèdre rhombique. La conservation de la structure cristalline ainsi que la stabilité particulière du dodécaèdre rhombique sont discutées en relation avec les propriétés de la liaison ionique. Les propriétés de luminescence de Gd2O3 dopé Eu3+ ont permis de mettre en évidence une transition de structure pour les agrégats de très petite taille. Cette transition peut être expliquée par la pression de Gibbs induite par la tension de surface. La simulation de la luminescence des agrégats montre l'influence importante de la surface dans l'élargissement inhomogène des spectres de luminescence des nanoparticules. Le confinement quantique dans Gd2O3 dopé Eu3+ est démontré. Le comportement observé présente des similitudes avec celui de matériaux moins ioniques. Une interprétation est proposée.
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Medel, Robert. "Schwingungsspektroskopische Untersuchungen zur Chiralitätserkennung und Torsionsdynamik bei Alkoholen". Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2020. http://hdl.handle.net/21.11130/00-1735-0000-0005-14BC-1.
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Bittmann, Léa. "Quantum Grothendieck rings, cluster algebras and quantum affine category O". Thesis, Sorbonne Paris Cité, 2019. http://www.theses.fr/2019USPCC024.
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L'objectif de cette thèse est de construire et d'étudier une structure d'anneau de Grothendieck quantique pour une catégorie O de représentations de la sous-algèbre de Borel Uq(b) d'une algèbre affine quantique Uq(g). On s'intéresse dans un premier lieu à la construction de modules standards asymptotiques pour la catégorie O, qui sont des analogues des modules standards existant dans la catégorie des représentations de dimension finie de Uq(^g). Une construction complète de ces modules est proposée dans le cas où l'algèbre de Lie simple sous-jacente g est sl2. Ensuite, nous définissons un tore quantique qui étend le tore quantique contenant l'anneau de Grothendieck quantique de la catégorie des représentations de dimension finie.Nous utilisons pour cela des notions liées aux algèbres amassées quantiques. Dans le même esprit, nous proposons une construction d'une structure d'algèbre amassée quantique sur l'anneau de Grothendieck quantique Kt(Cz) d'une sous-catégorie monoïdale Cz de la catégorie des représentations de dimension finie. Puis, nous définissons un anneau de Grothendieck quantique Kt(O+Z) d'une sous catégorie O+Z de la catégorie O, comme une algèbre amassée quantique. Nous établissons ensuite que cet anneau de Grothendieck quantique contient celui de la catégorie des représentations de dimension finie. Ce résultat est montré directement en type A, puis en tout type simplement lacé en utilisant la structure d'algèbre amassée quantique de Kt(CZ).Enfin, nous définissons des (q,t)-caractères pour des représentations simples de dimension infinie remarquables de la catégorie O. Ceci nous permet d'écrire des versions t-déformées de relations importantes dans l'anneau de Grothendieck classique de la catégorie O+Z qui ont des liens avec les systèmes intégrables quantiques associésThe aim of this thesis is to construct and study some quantum Grothendieck ring structure for the category O of representations of the Borel subalgebra Uq(^b) of a quantum affine algebra Uq(^g). First of all, we focus on the construction of asymptotical standard modules, analogs in the context of the category O of the standard modules in the category of finite-dimensional Uq(^g)-modules. A construction of these modules is given in the case where the underlying simple Lie algebra g is sl2. Next, we define a new quantum torus, which extends the quantum torus containing the quantum Grothendieck ring of the category of finite-dimensional modules. In order todo this, we use notions linked to quantum cluster algebras. In the same spirit, we build a quantum cluster algebra structure on the quantum Grothendieck ring of a monoidal subcategory CZ of the category of finite-dimensional representations. With this quantum torus, we de_ne the quantum Grothendieck ring Kt(O+Z) of a subcategory O+Z of the category O as a quantum cluster algebra. Then, we prove that this quantum Grothendieck ring contains that of the category of finite-dimensional representation. This result is first shown directly in type A, and then in all simply-laced types using the quantum cluster algebra structure of Kt(CZ). Finally, we define (q,t)-characters for some remarkable infinite-dimensional simple representations in the category O+Z. This enables us to write t-deformed analogs of important relations in the classical Grothendieck ring of the category O, which are related to the corresponding quantum integrable systems
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Hajdusek, Michal. "Thermodynamics, quantum computation and cluster states". Thesis, University of Leeds, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.531532.
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Raussendorf, Robert. "Measurement-based quantum computation with cluster states". Diss., lmu, 2003. http://nbn-resolving.de/urn:nbn:de:bvb:19-13674.
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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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Joo, Jaewoo. "Atomic and Optical Realizations of Cluster Quantum Computation". Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.484425.
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Beard, Bernard Borodel. "Improvements in cluster algorithms for quantum spin systems". Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/38414.
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Fish, C. D. "Connected quantized Weyl algebras and quantum cluster algebras". Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/16636/.
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We investigate a class of noncommutative algebras, which we call connected quantized Weyl algebras, with a simple description in terms of generators and relations. We already knew of two families, both of which arise from cluster mutation in mutationperiodic quivers, and we show that for generic values of a scalar parameter q these are the only examples. We then investigate the ring-theoretic properties of these two families, determining their prime spectra, automorphism groups and some results on their Krull and global dimensions. The theory of ambiskew polynomial rings and generalised Weyl algebras is useful here and we obtain a description of the height 1 prime ideals in certain generalised Weyl algebras, along with some results on the dimension theory of these rings. We also investigate the semiclassical limit Poisson algebras of the connected quantized Weyl algebras, and compare the prime spectra and Poisson prime spectra of the corresponding rings. We also show that the quantum cluster algebra without coefficients for an acyclic quiver is simple, and extend this result to find a simple localisation in the case where there are coefficients. Finally, we investigate quantum cluster algebra structures related to the connected quantized Weyl algebras discussed earlier, and use these to illustrate the previous result.
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Fujii, Keisuke. "Fault-Tolerant Quantum Computation on Logical Cluster States". 京都大学 (Kyoto University), 2011. http://hdl.handle.net/2433/142173.
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