Articles de revues : « Electronic Structure Calculations - Computational Methods »

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Wang, Lin-Wang. « Novel Computational Methods for Nanostructure Electronic Structure Calculations ». Annual Review of Physical Chemistry 61, n^o 1 (mars 2010) : 19–39. http://dx.doi.org/10.1146/annurev.physchem.012809.103344.

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Zhang, Xin, Jinwei Zhu, Zaiwen Wen et Aihui Zhou. « Gradient Type Optimization Methods For Electronic Structure Calculations ». SIAM Journal on Scientific Computing 36, n^o 3 (janvier 2014) : C265—C289. http://dx.doi.org/10.1137/130932934.

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Richie, D. A., P. von Allmen, K. Hess et Richard M. Martin. « Electronic Structure Calculations Using An Adaptive Wavelet Basis ». VLSI Design 8, n^o 1-4 (1 janvier 1998) : 159–63. http://dx.doi.org/10.1155/1998/62853.

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The use of a wavelet basis can lead to efficient methods for performing ab initio electronic structure calculations of inherently localized structures. In this work wavelets are used to construct an adaptive basis which is optimized dynamically throughout the calculation. The computational effort of such a method should scale linearly with the number of basis functions. The adaptive basis is tested for the case of bulk Si using only a local s-pseudopotential.

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Barettin, D., S. Madsen, B. Lassen et M. Willatzen. « Computational Methods for Electromechanical Fields in Self-Assembled Quantum Dots ». Communications in Computational Physics 11, n^o 3 (mars 2012) : 797–830. http://dx.doi.org/10.4208/cicp.111110.110411a.

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AbstractA detailed comparison of continuum and valence force field strain calculations in quantum-dot structures is presented with particular emphasis to boundary conditions, their implementation in the finite-element method, and associated implications for electronic states. The first part of this work provides the equation framework for the elastic continuum model including piezoelectric effects in crystal structures as well as detailing the Keating model equations used in the atomistic valence force field calculations. Given the variety of possible structure shapes, a choice of pyramidal, spherical and cubic-dot shapes is made having in mind their pronounced shape differences and practical relevance. In this part boundary conditions are also considered; in particular the relevance of imposing different types of boundary conditions is highlighted and discussed.

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Zeng, Xiongzhi, Wei Hu, Xiao Zheng, Jin Zhao, Zhenyu Li et Jinlong Yang. « Computational characterization of nanosystems ». Chinese Journal of Chemical Physics 35, n^o 1 (février 2022) : 1–15. http://dx.doi.org/10.1063/1674-0068/cjcp2111233.

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Nanosystems play an important role in many applications. Due to their complexity, it is challenging to accurately characterize their structure and properties. An important means to reach such a goal is computational simulation, which is grounded on ab initio electronic structure calculations. Low scaling and accurate electronic-structure algorithms have been developed in recent years. Especially, the efficiency of hybrid density functional calculations for periodic systems has been significantly improved. With electronic structure information, simulation methods can be developed to directly obtain experimentally comparable data. For example, scanning tunneling microscopy images can be effectively simulated with advanced algorithms. When the system we are interested in is strongly coupled to environment, such as the Kondo effect, solving the hierarchical equations of motion turns out to be an effective way of computational characterization. Furthermore, the first principles simulation on the excited state dynamics rapidly emerges in recent years, and nonadiabatic molecular dynamics method plays an important role. For nanosystem involved chemical processes, such as graphene growth, multiscale simulation methods should be developed to characterize their atomic details. In this review, we review some recent progresses in methodology development for computational characterization of nanosystems. Advanced algorithms and software are essential for us to better understand of the nanoworld.

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Bligaard, Thomas, Martin P. Andersson, Karsten W. Jacobsen, Hans L. Skriver, Claus H. Christensen et Jens K. Nørskov. « Electronic-Structure-Based Design of Ordered Alloys ». MRS Bulletin 31, n^o 12 (décembre 2006) : 986–90. http://dx.doi.org/10.1557/mrs2006.225.

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AbstractWe describe some recent advances in the methodology of using electronic structure calculations for materials design. The methods have been developed for the design of ordered metallic alloys and metal alloy catalysts, but the considerations we present are relevant for the atomic-scale computational design of other materials as well. A central problem is how to treat the huge number of compounds that can be envisioned by varying the concentrations and the number of the elements involved. We discuss various strategies for approaching this problem and show how one strategy has led to the computational discovery of a promising catalytic metal alloy surface with high reactivity and low cost.

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Pototschnig, Johann V., Kenneth G. Dyall, Lucas Visscher et André Severo Pereira Gomes. « Electronic spectra of ytterbium fluoride from relativistic electronic structure calculations ». Physical Chemistry Chemical Physics 23, n^o 39 (2021) : 22330–43. http://dx.doi.org/10.1039/d1cp03701c.

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Potential energy curves for the YbF obtained by relativistic electronic structure methods are presented. Due to the difficulties of describing this system separate computations for open and closed f-shells were necessary.

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Stöhr, Martin, Troy Van Voorhis et Alexandre Tkatchenko. « Theory and practice of modeling van der Waals interactions in electronic-structure calculations ». Chemical Society Reviews 48, n^o 15 (2019) : 4118–54. http://dx.doi.org/10.1039/c9cs00060g.

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Opening the black box of van der Waals-inclusive electronic structure calculations: a tutorial-style introduction to van der Waals dispersion interactions, state-of-the-art methods in computational modeling and complementary experimental techniques.

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Breczko, T., V. Barkaline et J. Tamuliene. « INVESTIGATION OF GEOMETRIC AND ELECTRONIC STRUCTURES OF HEUSLER ALLOYS : CUBIC AND TETRAGONAL LATTICES ». EPH - International Journal of Applied Science 6, n^o 1 (27 mars 2020) : 1–5. http://dx.doi.org/10.53555/eijas.v6i1.102.

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Ni2MnGa and Co2MnGa compounds were investigated by using state-of-the-art computational ab-initio methods. The total energy calculations for the cubic and the tetrahedral structures, band structure together with suspensibility investigations were performed. The results of our investigations exhibited the dependence of magnetic properties of the compounds on their geometrical structure. The influence of Co and Ni on the magnetic properties of the compounds was disclosed, too.

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Fujiki, Ryo, Toru Matsui, Yasuteru Shigeta, Haruyuki Nakano et Norio Yoshida. « Recent Developments of Computational Methods for pKa Prediction Based on Electronic Structure Theory with Solvation Models ». J 4, n^o 4 (10 décembre 2021) : 849–64. http://dx.doi.org/10.3390/j4040058.

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The protonation/deprotonation reaction is one of the most fundamental processes in solutions and biological systems. Compounds with dissociative functional groups change their charge states by protonation/deprotonation. This change not only significantly alters the physical properties of a compound itself, but also has a profound effect on the surrounding molecules. In this paper, we review our recent developments of the methods for predicting the Ka, the equilibrium constant for protonation reactions or acid dissociation reactions. The pKa, which is a logarithm of Ka, is proportional to the reaction Gibbs energy of the protonation reaction, and the reaction free energy can be determined by electronic structure calculations with solvation models. The charge of the compound changes before and after protonation; therefore, the solvent effect plays an important role in determining the reaction Gibbs energy. Here, we review two solvation models: the continuum model, and the integral equation theory of molecular liquids. Furthermore, the reaction Gibbs energy calculations for the protonation reactions require special attention to the handling of dissociated protons. An efficient method for handling the free energy of dissociated protons will also be reviewed.

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Bao, Gang, Guanghui Hu et Di Liu. « Towards Translational Invariance of Total Energy with Finite Element Methods for Kohn-Sham Equation ». Communications in Computational Physics 19, n^o 1 (janvier 2016) : 1–23. http://dx.doi.org/10.4208/cicp.190115.200715a.

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AbstractNumerical oscillation of the total energy can be observed when the Kohn- Sham equation is solved by real-space methods to simulate the translational move of an electronic system. Effectively remove or reduce the unphysical oscillation is crucial not only for the optimization of the geometry of the electronic structure, but also for the study of molecular dynamics. In this paper, we study such unphysical oscillation based on the numerical framework in [G. Bao, G. H. Hu, and D. Liu, An h-adaptive finite element solver for the calculations of the electronic structures, Journal of Computational Physics, Volume 231, Issue 14, Pages 4967–4979, 2012], and deliver some numerical methods to constrain such unphysical effect for both pseudopotential and all-electron calculations, including a stabilized cubature strategy for Hamiltonian operator, and an a posteriori error estimator of the finite element methods for Kohn-Sham equation. The numerical results demonstrate the effectiveness of our method on restraining unphysical oscillation of the total energies.

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Gao, Weiwei, Linda Hung, Serdar Ogut et James R. Chelikowsky. « The stability, electronic structure, and optical absorption of boron-nitride diamondoids predicted with first-principles calculations ». Physical Chemistry Chemical Physics 20, n^o 28 (2018) : 19188–94. http://dx.doi.org/10.1039/c8cp02377h.

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Perrella, Fulvio, Federico Coppola, Nadia Rega et Alessio Petrone. « An Expedited Route to Optical and Electronic Properties at Finite Temperature via Unsupervised Learning ». Molecules 28, n^o 8 (12 avril 2023) : 3411. http://dx.doi.org/10.3390/molecules28083411.

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Electronic properties and absorption spectra are the grounds to investigate molecular electronic states and their interactions with the environment. Modeling and computations are required for the molecular understanding and design strategies of photo-active materials and sensors. However, the interpretation of such properties demands expensive computations and dealing with the interplay of electronic excited states with the conformational freedom of the chromophores in complex matrices (i.e., solvents, biomolecules, crystals) at finite temperature. Computational protocols combining time dependent density functional theory and ab initio molecular dynamics (MD) have become very powerful in this field, although they require still a large number of computations for a detailed reproduction of electronic properties, such as band shapes. Besides the ongoing research in more traditional computational chemistry fields, data analysis and machine learning methods have been increasingly employed as complementary approaches for efficient data exploration, prediction and model development, starting from the data resulting from MD simulations and electronic structure calculations. In this work, dataset reduction capabilities by unsupervised clustering techniques applied to MD trajectories are proposed and tested for the ab initio modeling of electronic absorption spectra of two challenging case studies: a non-covalent charge-transfer dimer and a ruthenium complex in solution at room temperature. The K-medoids clustering technique is applied and is proven to be able to reduce by ∼100 times the total cost of excited state calculations on an MD sampling with no loss in the accuracy and it also provides an easier understanding of the representative structures (medoids) to be analyzed on the molecular scale.

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Gibbs, Josh, Alberto Otero de la Roza, Adam Johan Bergren et Gino A. DiLabio. « Interpretation of molecular device transport calculations ». Canadian Journal of Chemistry 94, n^o 12 (décembre 2016) : 1022–27. http://dx.doi.org/10.1139/cjc-2016-0279.

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The field of molecular electronics will benefit from rational design approaches based on a complete understanding of the electronic structure of molecule-based devices. However, many computational approaches that are used to study molecular-scale devices are based on methods that have deficiencies that must be understood in order for those methods to be useful to the modeling and experimental community. Density-functional theory based methods have some well-known pitfalls that limit their application to the study of electron transport in models of molecular junction devices. Some of the impacts of these deficiencies are highlighted in this work through the use of a graphene model system and a variety of simple hydrocarbon molecules. Self-interaction error in simple functionals built from the local density approximation and the generalized gradient approximation results in very large errors in predicted absolute and relative ionization potentials. This demonstrates that electron transmission spectra predicted using these functionals should be considered with caution. We also demonstrate that care must be taken with the use of finite models for electrodes.

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Beran, Gregory. « Modeling molecular crystals with fragment-based electronic structure techniques ». Acta Crystallographica Section A Foundations and Advances 70, a1 (5 août 2014) : C1616. http://dx.doi.org/10.1107/s2053273314083831.

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"A proper theoretical description of molecular crystal packing requires a uniformly high-quality treatment of the diverse intra- and intermolecular interactions. Fragment-based electronic structure methods enable the application of accurate electronic structure approaches to chemically interesting molecular crystals by decomposing the total crystal energy into the sum of many smaller ""fragment"" calculations. In this talk, we will discuss (1) the hybrid quantum/classical fragment based approach we have developed for molecular crystal problems, (2) state-of-the-art electronic structure approaches for treating the individual fragments with the requisite accuracy and acceptable computational effort, and (3) applications of these techniques to interesting molecular crystal problems."

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Saßnick, Holger-Dietrich, et Caterina Cocchi. « Exploring cesium–tellurium phase space via high-throughput calculations beyond semi-local density-functional theory ». Journal of Chemical Physics 156, n^o 10 (14 mars 2022) : 104108. http://dx.doi.org/10.1063/5.0082710.

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Boosted by the relentless increase in available computational resources, high-throughput calculations based on first-principles methods have become a powerful tool to screen a huge range of materials. The backbone of these studies is well-structured and reproducible workflows efficiently returning the desired properties given chemical compositions and atomic arrangements as sole input. Herein, we present a new workflow designed to compute the stability and the electronic properties of crystalline materials from density-functional theory using the strongly constrained and appropriately normed approximation (SCAN) for the exchange–correlation potential. We show the performance of the developed tool exploring the binary Cs–Te phase space that hosts cesium telluride, a semiconducting material widely used as a photocathode in particle accelerators. Starting from a pool of structures retrieved from open computational material databases, we analyze formation energies as a function of the relative Cs content and for a few selected crystals, we investigate the band structures and density of states unraveling interconnections among the structure, stoichiometry, stability, and electronic properties. Our study contributes to the ongoing research on alkali-based photocathodes and demonstrates that high-throughput calculations based on state-of-the-art first-principles methods can complement experiments in the search for optimal materials for next-generation electron sources.

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Slipokurov, V. A., P. P. Korniychuk et A. V. Zinovchuk. « A method for fast calculating the electronic states in 2D quantum structures based on AIIIBV nitrides ». Semiconductor Physics, Quantum Electronics and Optoelectronics 26, n^o 2 (26 juin 2023) : 165–72. http://dx.doi.org/10.15407/spqeo26.02.165.

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The paper presents a method for fast calculating the electronic states in two-dimensional quantum structures based on AIIIBV nitrides. The method is based on the representation of electronic states in the form of a linear combination of bulk wave functions of materials, from which quantum structures are made. The parameters and criteria for the selection of bulk wave functions that provides fast convergence of the numerical procedures for calculating the eigenvalues of the quantum Hamiltonian have been considered. The results of the calculations have been given both for one polar InGaN/GaN quantum well and for a system of several quantum wells. Being based on the full band structure of AIIIBV nitrides with a wurtzite-type crystal lattice, the proposed approach takes into account the states far from the center of the Brillouin zone, while preserving the computational efficiency of traditional methods of envelope function in approximating the effective mass.

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Khoromskaia, Venera. « Black-Box Hartree–Fock Solver by Tensor Numerical Methods ». Computational Methods in Applied Mathematics 14, n^o 1 (1 janvier 2014) : 89–111. http://dx.doi.org/10.1515/cmam-2013-0023.

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Abstract. The Hartree–Fock eigenvalue problem governed by the 3D integro-differential operator is the basic model in ab initio electronic structure calculations. Several years ago the idea to solve the Hartree–Fock equation by a fully 3D grid based numerical approach seemed to be a fantasy, and the tensor-structured methods did not exist. In fact, these methods evolved during the work on this challenging problem. In this paper, our recent results on the topic are outlined and the black-box Hartree–Fock solver by tensor numerical methods is presented. The approach is based on the rank-structured calculation of the core hamiltonian and of the two-electron integrals tensor using the problem adapted basis functions discretized on $n\times n\times n$ 3D Cartesian grids. The arising 3D convolution transforms with the Newton kernel are replaced by a combination of 1D convolutions and 1D Hadamard and scalar products. The approach allows huge spatial grids, with $n^3\simeq 10^{15}$, yielding high resolution at low cost. The two-electron integrals are computed via multiple factorizations. The Laplacian Galerkin matrix can be computed “on-the-fly” using the quantized tensor approximation of $O(\log n)$ complexity. The performance of the black-box solver in Matlab implementation is compatible with the benchmark packages based on the analytical (pre)evaluation of the multidimensional convolution integrals. We present ab initio Hartree–Fock calculations of the ground state energy for the amino acid molecules, and of the “energy bands” for the model examples of extended (quasi-periodic) systems.

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Drougas, Evangelos, et Agnie M. Kosmas. « Computational investigation of isomeric and conformeric structures of methyl iodoperoxide ». Canadian Journal of Chemistry 83, n^o 1 (1 janvier 2005) : 9–15. http://dx.doi.org/10.1139/v04-156.

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Quantum mechanical electronic structure methods are employed to investigate the isomeric and conformeric stuctures of methyl iodoperoxide. Optimized geometries and harmonic vibrational frequencies are calculated at the MP2 level of theory using two types of basis sets, the 6-311G(d,p) for all atoms and the 6-311G(d,p) combined with the LANL2DZ relativistic ECP procedure for iodine. Refinement of the energetics has been accomplished by performing single-point CCSD(T) calculations. Five isomers were determined in total among which iodomethyl hydroperoxide (ICH2OOH) is found to be the lowest energy structure. Conformational barriers and transition states that connect the isomeric forms have been characterized.Key words: methyl iodoperoxide, isomers, conformers.

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Anaya-Morales, A., et F. Delgado. « Enquiring Electronic Structure Using Quantum Computers : Hands on Qiskit ». Journal of Physics : Conference Series 2448, n^o 1 (1 février 2023) : 012014. http://dx.doi.org/10.1088/1742-6596/2448/1/012014.

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Abstract Solving the electronic structure for multi-electronic systems is a hard problem. Even for small atoms and molecules, approximations have to be made in order to solve numerically the Schrödinger equation. Although different methods have been developed to take into account electron correlations, their computational cost reduces their feasibility. Quantum simulation provides an alternative to traditional computational methods for enquiring the electronic structure of molecules. Specifically, the Variational Quantum Eigensolver (VQE) algorithm provides a hybrid quantum-classical algorithm for the implementation on current near term quantum devices. In this work, we explore the implementation of VQE on Qiskit for calculating the ground-state energy of diatomic Hydrogen molecule.

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Epifanovsky, Evgeny, Michael Wormit, Tomasz Kuś, Arie Landau, Dmitry Zuev, Kirill Khistyaev, Prashant Manohar, Ilya Kaliman, Andreas Dreuw et Anna I. Krylov. « New implementation of high-level correlated methods using a general block tensor library for high-performance electronic structure calculations ». Journal of Computational Chemistry 34, n^o 26 (10 juillet 2013) : 2293–309. http://dx.doi.org/10.1002/jcc.23377.

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Albaitai, Asmaa, et Saifaldeen M. Abdalhadi. « Modelling technique trend (interatomic potential) to study the mineral surfaces : Review ». Samarra Journal of Pure and Applied Science 2, n^o 3 (22 septembre 2021) : 62–73. http://dx.doi.org/10.54153/sjpas.2020.v2i3.99.

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Computational chemistry is another branch of chemistry that can be used to model the material which is based on the mathematical methods and combined that with the theories of the quantum mechanics. However, in this filed there are two different techniques or categories, classical interatomic potential and the electronic structure methodology. The aim of this paper is to describe how can modelling the structures and energetics of surface and interface processes of minerals surface, using the classical atomistic simulation methods. We will illustrate the types of potentials and some of Codes (Gulp and METADISE) which is needed to do these calculations to elucidate the structures and stabilities as well.

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Raja, G., K. Saravanan et S. Sivakumar. « Molecular and Electronic Structure of 1-Naphtol : Ab Initio Molecular Orbital and Density Functional Study ». Applied Mechanics and Materials 110-116 (octobre 2011) : 1862–69. http://dx.doi.org/10.4028/www.scientific.net/amm.110-116.1862.

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The molecular vibrations of 1-Naphtol were investigated in polycrystalline sample, at room temperature, by FT- IR and FT-Raman spectroscopy. In parallel, ab initio and various density functional (DFT) methods were used to determine the geometrical, energetic and vibrational characteristics of 1-Naphtol . On the basis of B3LYP/6-31G* and B3LYP/6-311+G** methods and basis set combinations, a xnormal mode analysis was performed to assign the various fundamental frequencies according to the total energy distribution (TED). The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical force field. The Infrared and Raman spectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. Simulation of Infrared and Raman spectra, utilizing the results of these calculations led to excellent overall agreement with observed spectral patterns. The investigation is performed using quantum chemical calculations conducted by means of the Gaussian 98W and Guassview set of programs. Further, density functional theory (DFT) combined with quantum chemical calculations to determine the first-order hyperpolarizability.

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Muchall, Heidi M., Nick H. Werstiuk, Jiangong Ma, Thomas T. Tidwell et Kuangsen Sung. « Conformational behavior and electronic structure of silylketenes studied with quantum chemical calculations and photoelectron spectroscopy ». Canadian Journal of Chemistry 75, n^o 12 (1 décembre 1997) : 1851–61. http://dx.doi.org/10.1139/v97-618.

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The He(I) photoelectron spectra of silylketenes (Me3Si)2C=C=O (1), Me5Si2CH=C=O (2), Me2Si(CH=C=O)2 (3), MeSi(CH=C=O)3 (4), (SiMe2CH=C=O)2 (5), and (CH2SiMe2CH=C=O)2 (6) have been recorded and their structures and orbital energies have been calculated by ab initio methods. Orbital energies for disilanes 2 and 5 are strongly dependent on a Si-Si-C-C torsional angle due to σ–π orbital interaction. Comparisons between experimental and simulated spectra show that 2 and 5 prefer conformations in which the Si—Si bond and ketene group(s) are approximately orthogonal (113° and 111°, respectively). Silylalkenes Me5Si2CH=CH2 (7) and (SiMe2CH=CH2)2 (8), which have been included in the computational study, show the same behavior as their corresponding silylketenes. Silylbis- and trisketenes 3–6 do not exhibit π–π interaction of any significance. For Si—Si containing compounds, the best agreement between experimental and computed data was obtained when Becke3LYP/6-31G*//HF/3-21G* was employed. Keywords: conformational behavior, electronic structure, photoelectron spectroscopy, quantum chemical calculations, silylketenes.

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Kirchner-Hall, Nicole E., Wayne Zhao, Yihuang Xiong, Iurii Timrov et Ismaila Dabo. « Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps ». Applied Sciences 11, n^o 5 (8 mars 2021) : 2395. http://dx.doi.org/10.3390/app11052395.

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Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost.

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Malcherek, Thomas, Markus Borowski et Anne Bosenick. « Structure and phase transitions of CaTaOAlO4 ». Journal of Applied Crystallography 37, n^o 1 (17 janvier 2004) : 117–22. http://dx.doi.org/10.1107/s002188980302689x.

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The structure of CaTaOAlO4(CTAO) has been determined using X-ray powder diffraction and density functional methods in combination with27Al MAS NMR spectroscopy. A structural phase transition occurs near room temperature in CTAO as indicated by heat-capacity measurements, lattice strain data and infrared spectroscopy. Rietveld analysis of the powder diffraction data does not indicate deviation from monoclinic symmetryC2/c. But the observed quadrupolar coupling of the Al atom is reproduced by the electronic structure calculations only in a structure with space-group symmetryP21/n, distinguished by two different Ta coordination environments. The atomic coordinates of this low-temperature structure of CTAO are obtained by computational force relaxation within the experimental unit cell determined at 170 K.

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Paolone, Annalisa, et Sergio Brutti. « Comparison of the Performances of Different Computational Methods to Calculate the Electrochemical Stability of Selected Ionic Liquids ». Materials 14, n^o 12 (10 juin 2021) : 3221. http://dx.doi.org/10.3390/ma14123221.

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The electrochemical stability windows (ESW) of selected ionic liquids have been calculated by comparing different computational approaches previously suggested in the literature. The molecular systems under study are based on di-alkyl imidazolium and tetra-alkyl ammonium cations coupled with two different imide anions (namely, bis-fluorosulfonyl imide and bis-trifluoromethyl sulfonyl imide), for which an experimental investigation of the ESW is available. Thermodynamic oxidation and reduction potentials have here been estimated by different models based on calculations either on single ions or on ionic couples. Various Density Functional Theory (DFT) functionals (MP2, B3LYP, B3LYP including a polarizable medium and empirical dispersion forces) were exploited. Both vertical and adiabatic transitions between the starting states and the oxidized or reduced states were considered. The approach based on calculations on ionic couples is not able to reproduce the experimental data, whatever the used DFT functional. The best quantitative agreement is obtained by calculations on single ions when the MP2 functional in vacuum is considered and the transitions between differently charged states are vertical (purely electronic without the relaxation of the structure). The B3LYP functional underestimates the ESW. The inclusion of a polar medium excessively widens the ESW, while a large shrinkage of the ESW is obtained by adopting an adiabatic transition scheme instead of a vertical transition one.

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Stefaniu, Amalia, Valeria Gabriela Savoiu, Irina Lupescu et Olga Iulian. « Computational study on 3D structure of L-aspartic acid and L-glutamic acid : molecular descriptors and properties ». Ovidius University Annals of Chemistry 27, n^o 1 (1 juin 2016) : 48–52. http://dx.doi.org/10.1515/auoc-2016-0012.

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Abstract The aim of this work is to provide a comprehensive and complex analysis of molecular descriptors and properties of two similar amino acids, L-Aspartic acid and L-Glutamic acid, using a software tool for calculations and properties predictions. As amino acids are model compounds for predicting the physical-chemical properties and behavior of biological, larger molecules as peptides or proteins, researches were focused on providing accurate mechanical calculations using: molecular/mechanical methods. Our study aims to initiate a linear scaling approach, by dividing a large system into small subsystems and performing the calculations for each, individually, then, embedding and correcting the information globally. The calculations were performed on the 3D structure of the studied amino acids that were first generated, as CPK model, and optimized by energy minimization. A comparative assay on their topological, molecular descriptors and properties was conducted, in vacuum and in water, using the Hartree-Fock model and second-order Møller-Plesset perturbation theory MP2 for predicting structure, energy and property calculations with Spartan’14 software. Values of molecular properties such as area, volume, polar surface area, polarizability, ovality, logP, dipole moment, HOMO-LUMO gap, distances and angles between atoms, were obtained. The results have been interpreted in terms of electronic effects of side chain groups, molecular deformability, steric factors and reactivity. This approach can be extended to other amino acids in order to predict protein-ligand interactions, important aspects in drug design studies and protein engineering.

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Mihm, Tina N., Tobias Schäfer, Sai Kumar Ramadugu, Laura Weiler, Andreas Grüneis et James J. Shepherd. « A shortcut to the thermodynamic limit for quantum many-body calculations of metals ». Nature Computational Science 1, n^o 12 (décembre 2021) : 801–8. http://dx.doi.org/10.1038/s43588-021-00165-1.

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AbstractComputationally efficient and accurate quantum mechanical approximations to solve the many-electron Schrödinger equation are crucial for computational materials science. Methods such as coupled cluster theory show potential for widespread adoption if computational cost bottlenecks can be removed. For example, extremely dense k-point grids are required to model long-range electronic correlation effects, particularly for metals. Although these grids can be made more effective by averaging calculations over an offset (or twist angle), the resultant cost in time for coupled cluster theory is prohibitive. We show here that a single special twist angle can be found using the transition structure factor, which provides the same benefit as twist averaging with one or two orders of magnitude reduction in computational time. We demonstrate that this not only works for metal systems but also is applicable to a broader range of materials, including insulators and semiconductors.

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Redfern, Simon A. T. « Advances in computer modelling of mineral properties ». Mineralogical Magazine 59, n^o 397 (décembre 1995) : 585–87. http://dx.doi.org/10.1180/minmag.1995.059.397.01.

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The following six papers were presented at a meeting, held in September 1994, which reviewed some of the recent advances in the application of computational methods to mineralogy. Talks covered the developing and challenging field of ab initio quantum mechanical computations as well as new applications and insights afforded by the use and refinement of the more established methods of empirical simulation and modelling. The former attempt to solve Schrdinger's equation for the material in question, and in doing so determine the energy surface and electronic structure. The latter use parameterized interatomic potentials to describe the energy of interaction between pairs or groups of atoms, the parameters typically derived either by fitting to the results of quantum mechanical calculations of small clusters, or empirically determined from fitting to the known physical properties of crystals (e.g. to the elastic constants, structural parameters or dielectric constants). A certain rivalry and antipathy occasionally surfaces between computational theorists from either camp.

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Rodrigues, Edson Silvio Batista, Isaac Yves Lopes de Macêdo, Larissa Lesley da Silva Lima, Douglas Vieira Thomaz, Carlos Eduardo Peixoto da Cunha, Mayk Teles de Oliveira, Nara Ballaminut et al. « Electrochemical Characterization of Central Action Tricyclic Drugs by Voltammetric Techniques and Density Functional Theory Calculations ». Pharmaceuticals 12, n^o 3 (1 août 2019) : 116. http://dx.doi.org/10.3390/ph12030116.

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This work details the study of the redox behavior of the drugs cyclobenzaprine (CBP), amitriptyline (AMP) and nortriptyline (NOR) through voltammetric methods and computational chemistry. Results obtained in this study show that the amine moiety of each compound is more likely to undergo oxidation at 1a at Ep1a ≈ 0.69, 0.79, 0.93 V (vs. Ag/AgCl/KClsat) for CBP, AMP and NOR, respectively. Moreover, CBP presented a second peak, 2a at Ep2a ≈ 0.98 V (vs. Ag/AgCl/KClsat) at pH 7.0. Furthermore, the electronic structure calculation results corroborate the electrochemical assays regarding the HOMO energies of the lowest energy conformers of each molecule. The mechanism for each anodic process is proposed according to electroanalytical and computational chemistry findings, which show evidence that the methods herein employed may be a valuable alternative to study the redox behavior of structurally similar drugs.

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WIJESEKERA, NIMAL, GUOGANG FENG et THOMAS L. BECK. « MULTISCALE ALGORITHMS FOR EIGENVALUE PROBLEMS ». Journal of Theoretical and Computational Chemistry 02, n^o 04 (décembre 2003) : 553–61. http://dx.doi.org/10.1142/s0219633603000665.

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Iterative multiscale methods for electronic structure calculations offer several advantages for large-scale problems. Here we examine a nonlinear full approximation scheme (FAS) multigrid method for solving fixed potential and self-consistent eigenvalue problems. In principle, the expensive orthogonalization and Ritz projection operations can be moved to coarse levels, thus substantially reducing the overall computational expense. Results of the nonlinear multiscale approach are presented for simple fixed potential problems and for self-consistent pseudopotential calculations on large molecules. It is shown that, while excellent efficiencies can be obtained for problems with small numbers of states or well-defined eigenvalue cluster structure, the algorithm in its original form stalls for large-molecule problems with tens of occupied levels. Work is in progress to attempt to alleviate those difficulties.

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Harbury, Henry K., et Wolfgang Porod. « Parallel Computation for Electronic Waves in Quantum Corrals ». VLSI Design 6, n^o 1-4 (1 janvier 1998) : 47–51. http://dx.doi.org/10.1155/1998/15645.

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Recent scanning tunneling microscopy (STM) studies on the (111) faces of noble metals have directly imaged electronic surface-confined states and dramatic standing-wave patterns have been observed 1,2]. We solve for the local density of electronic states in these “leaky” quantum corral confinement structures using a coherent elastic scattering theory. We seek solutions of the two-dimensional Schrödinger equation compatible with non-reflecting boundary conditions which asymptotically satisfy the Sommerfeld radiation condition [11,14]. The large matrices generated by the discretization of realistic quantum corral structures require the use of sparse matrix methods. In addition, a parallel finite element solution was undertaken using the message passing interface standard (MPI) and the Portable, Extensible, Toolkit for Scientific Computation (PETSc) [5] for an efficient computational solution on both distributed and shared memory architectures. Our calculations reveal excellent agreement with the reported experimental dl/dV STM data.

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Morgante, Pierpaolo, et Roberto Peverati. « Comparison of the Performance of Density Functional Methods for the Description of Spin States and Binding Energies of Porphyrins ». Molecules 28, n^o 8 (15 avril 2023) : 3487. http://dx.doi.org/10.3390/molecules28083487.

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This work analyzes the performance of 250 electronic structure theory methods (including 240 density functional approximations) for the description of spin states and the binding properties of iron, manganese, and cobalt porphyrins. The assessment employs the Por21 database of high-level computational data (CASPT2 reference energies taken from the literature). Results show that current approximations fail to achieve the “chemical accuracy” target of 1.0 kcal/mol by a long margin. The best-performing methods achieve a mean unsigned error (MUE) <15.0 kcal/mol, but the errors are at least twice as large for most methods. Semilocal functionals and global hybrid functionals with a low percentage of exact exchange are found to be the least problematic for spin states and binding energies, in agreement with the general knowledge in transition metal computational chemistry. Approximations with high percentages of exact exchange (including range-separated and double-hybrid functionals) can lead to catastrophic failures. More modern approximations usually perform better than older functionals. An accurate statistical analysis of the results also casts doubts on some of the reference energies calculated using multireference methods. Suggestions and general guidelines for users are provided in the conclusions. These results hopefully stimulate advances for both the wave function and the density functional side of electronic structure calculations.

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Ibrir, Miloud. « Structural, electronic and thermoelectric properties of the intermetallic materials based on Mg2X (X= Si, Ge, Sn) : DFT calculations ». International Journal of Energetica 2, n^o 2 (31 décembre 2017) : 25. http://dx.doi.org/10.47238/ijeca.v2i2.44.

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The scope of this work is the investigation of the physical properties of chalcopyrite materials using ab-initio methods in order to simulate a new structure of thin-films photovoltaic cells with high conversion efficiency. In the first framework, we obtained the results of calculations based on Density Functional Theory (DFT) using the full-potential linearized augmented plane wave method (FP-LAPW) as involved in the WIEN2K computational package. For the exchange-correlation potential, the local density approximation (LDA) was used to calculate the lattice parameters, Bulk modulus and its first derivative as well as the densities of states of the intermetallic semiconductors materials based on Mg2X (X=Si, Ge and Sn). The semi-local Becke-Johnson (mBJ) potential and its modified form proposed by Tran and Blaha (TB-mBJ) were also used for studying the electronic and thermoelectric properties; (merit factor, Seebeck coefficient, electronic conductivity). The achieved results were compared to computational works and other data acquired experimentally.

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Pike, Nicholas A., Ruth Pachter, Alan D. Martinez et Gary Cook. « Computational analysis of the optical response of ZnSe with d-orbital defects ». Journal of Physics : Condensed Matter 34, n^o 20 (25 mars 2022) : 205402. http://dx.doi.org/10.1088/1361-648x/ac594a.

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Abstract The doping of wide band-gap semiconducting ZnSe by transition metal (TM) atoms finds applications from mid-infrared lasing, sensing, photoelectrochemical cells, to nonlinear optics. Yet understanding the response of these materials at the atomic and electronic level is lacking, particularly in comparing a range of TM dopants, which were studied primarily by phenomenological crystal-field theory. In this work, to investigate bulk ZnSe singly doped with first-row TM atoms, specifically Ti through Cu, we applied a first-principles approach and crystal-field theory to explain the origin of the infrared absorption. We show that the use of an appropriate exchange–correlation functional and a Hubbard U correction to account for electron correlation improved the determination of the electronic transitions in these systems. We outline an approach for the calculation of the crystal-field splitting from first-principles and find it useful in providing a measure of dopant effects, also in qualitative comparison to our experimental characterization for ZnSe doped with Fe, Cr, and Ni. Our calculated absorption spectra indicate absorption signatures in the mid-infrared range, while the absorption in the visible portion of the spectrum is attributed to the ZnSe host. Our calculations will potentially motivate further experimental exploration of TM-doped ZnSe. Finally, the methods used here provide a route towards computational high-throughput screening of TM dopants in III–V materials through a combination of the electronic band structure and crystal-field theory.

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Corà, Furio, Luis Gómez-Hortigüela et C. Richard A. Catlow. « Aerobic oxidation of hydrocarbons in Mn-doped aluminophosphates : a computational perspective to understand mechanism and selectivity ». Proceedings of the Royal Society A : Mathematical, Physical and Engineering Sciences 468, n^o 2143 (7 mars 2012) : 2053–69. http://dx.doi.org/10.1098/rspa.2012.0046.

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We discuss the mechanism and energetics for the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates with the AFI structure (Mn-APO-5), obtained computationally using electronic structure techniques. Calculations have been performed employing hybrid exchange density functional theory methods under periodic boundary conditions. The active sites of the catalyst are tetrahedral Mn ions isomorphously replacing Al in the microporous crystalline framework of the AlPO host. Since all Al sites in AFI are symmetry equivalent, all Mn dopants are in an identical chemical and structural environment, and hence satisfy the definition of a single-site heterogeneous catalyst. We focus in particular on the atomic-level origin of selectivity in this catalytic reaction.

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ONDOCKO, STEFAN, JOZEF SVETLIK, TOMAS STEJSKAL, MICHAL SASALA et LUKAS HRIVNIAK. « COMPARISON SELECTED NUMERICAL METHODS FOR THE CALCULATION INVERSE KINEMATICS OF NON-STANDARD MODULAR ROBOTIC ARM CONSISTING OF UNIQUE ROTATIONAL MODULES ». MM Science Journal 2021, n^o 2 (2 juin 2021) : 4468–73. http://dx.doi.org/10.17973/mmsj.2021_6_2021042.

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The paper compares the most commonly used numerical methods of solving a set of nonlinear equations, especially in terms of computational speed. The methods are applied to a set of nonlinear equations that describe the forward kinematics of a non-standard robotic arm. This arm is an open-loop kinematics chain, composed of special rotary modules. A non-standard feature of the modules is the unlimited rotation around their own axis. This robotic arm consists of six such modules and, thus, has six degrees of freedom. Computations of this nonlinear set of equations are also called inverse kinematics. All computations were performed in Matlab. The same initial conditions, the computation input parameters, and the same structure of the program was used with each method. By applying the below mentioned known methods to the same set, we sought to choose a suitable computation method for the given type of mechanism.

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Al-Sehemi, Abdullah G., Tarek M. El-Gogary, Karl Peter Wolschann et Gottfried Koehler. « Structure and Stability of Chemically Modified DNA Bases : Quantum Chemical Calculations on 16 Isomers of Diphosphocytosine ». ISRN Physical Chemistry 2013 (25 février 2013) : 1–10. http://dx.doi.org/10.1155/2013/146401.

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We studied for the first time 16 tautomers/rotamers of diphosphocytosine by four computational methods. Some of these tautomers/rotamers are isoenergetic although they have different structures. High-level electron correlation MP2 and MP4(SDQ) ab initio methods and density functional methods employing a B3LYP and the new M06-2X functional were used to study the structure and relative stability of 16 tautomers/rotamers of diphosphocytosine. The dienol tautomers of diphosphocytosine are shown to be much more stable than the keto-enol and diketo forms. The tautomers/rotamers stability could be ranked as PC3 = PC12 < PC2 = PC11 < PC1 < PC10 < PC8 < PC9 < PC15 < PC16 < PC6 ~ PC7 < PC13 < PC4 ~ PC14 < PC5. This stability order was discussed in the light of stereo and electronic factors. Solvation effect has been modeled in a high dielectric solvent, water using the polarized continuum model (PCM). Consideration of the solvent causes some reordering of the relative stability of diphosphocytosine tautomers: PC3 ~ PC12 ~ PC2 ~ PC11 < PC1 < PC10 < PC8 < PC9 < PC15 ~ PC16 < PC13 < PC6 ~ PC7 ~ PC14 < PC4 ~ PC5.

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Chan, Kwai S., Yi Ming Pan et Yi Der Lee. « First-Principles Computation of Transition-Metal Diffusion Mobility ». Defect and Diffusion Forum 266 (septembre 2007) : 73–82. http://dx.doi.org/10.4028/www.scientific.net/ddf.266.73.

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First–principle computational methods have been utilized to compute the diffusion mobility of Mo, Cr, Fe, and W. A local density-based full-potential linearized augmented plane wave (FLAPW) code, named WIEN2K, was utilized to compute the electronic structure and total energy of an n-atom supercell with atom positions designed to simulate the desired diffusion processes. The computational procedure involves the calculations of the energy for vacancy formation and the energy barrier for solute migration in the host metal. First-principles computational results of the energy of vacancy formation, solute migration energy, activation energy for self-diffusion, as well as diffusion of Mo, Cr, Fe, and W solutes in Ni and vice versa are presented and compared against experimental data from the literature.

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Sanna, Nico, et Maurizio Benfatto. « Benchmarking Plane Waves Quantum Mechanical Calculations of Iron(II) Tris(2,2′-bipyridine) Complex by X-ray Absorption Spectroscopy ». Condensed Matter 7, n^o 1 (27 janvier 2022) : 16. http://dx.doi.org/10.3390/condmat7010016.

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In this work, we used, for the first time, a computational Self-Consistent Field procedure based on plane waves to describe the low and high spin conformational states of the complex [Fe(bpy)3]2+. The results obtained in the study of the minimum energy structures of this complex, a prototype of a wide class of compounds called Spin Cross Over, show how the plane wave calculations are in line with the most recent studies based on gaussian basis set functions and, above all, reproduce within acceptable errors the experimental spectra of X-ray absorption near-edge structure spectroscopy (XANES). This preliminary study shows the capabilities of plane wave methods to correctly describe the molecular structures of metal-organic complexes of this type and paves the way for future even complex computational simulations based on the energy gradient, such as Nudge Elastic Band or ab-initio Born-Oppenheimer molecular dynamics.

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Abgaryan, Karine, Ilya Mutigullin et Dmitriy Bazhanov. « Multiscale Computational Model of Nitride Semiconductor Nanostructures ». Advanced Materials Research 560-561 (août 2012) : 1133–37. http://dx.doi.org/10.4028/www.scientific.net/amr.560-561.1133.

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Theoretical multiscale model of nitride semiconductor nanostructure is proposed. The model combines various computational methods such as density functional theory, molecular dynamics and kinetic Monte Carlo. As a first step of implementation of proposed approach ab initio calculations of structural and electronic properties of two different structures InN/Si and AlN/AlGaN/GaN heterostructures were carried out. In particular, the influence of oxygen on InN/Si adhesion energy was studied. AlN, GaN, AlxGa1-xN (x=0.33) spontaneous and piezoelectric polarizations as well as sheet carrier concentrations at GaN/AlGaN interface were calculated. Obtained value for sheet carrier concentration at GaN/AlGaN interface is close to experimental data.

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Male, Yusthinus T., Djulia Onggo, Muhamad A. Martoprawiro et Ismunandar Ismunandar. « THEORETICAL STUDY OF THE [Fe(en)₂(NCS)₂] COMPLEX WITH REPARAMETERIZED DENSITY FUNCTIONALS ». Indonesian Journal of Chemistry 9, n^o 3 (24 juin 2010) : 432–36. http://dx.doi.org/10.22146/ijc.21511.

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Quantum chemical studies have been carried out on the Fe(en)2(NCS)2 (en = ethylenediamine) complex both in low and high spin states (S = 0 and S = 2) using hybrid exchange-correlation functional (B3LYP) and non-hybrid method (BLYP). Calculations were performed in vacuum and in methanol to study the effect of cis-trans geometry on the structure and energy difference between low-spin (LS) and high-spin (HS) states of iron (II) complexes. Full geometry optimizations of the complexes show that hybrid method consistently gives higher energy difference between LS and HS states than the nonhybrid methods. Calculations with reparameterized density functional theory that showed more reasonable electronic energy splittings in previous research was also carried out. In addition, the computational study of Fe(en)2(NCS)2 in vacuum and methanol with PCM method showed that the complexes tend to adopt cis geometry. This geometry showed much less charge transfer in the substitutions of NCS- ligands compare to trans geometry. Keywords: Electronic structure, spin states, density functional, frontiers orbitals

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Harper, Angela F., Matthew L. Evans, James P. Darby, Bora Karasulu, Can P. Koçer, Joseph R. Nelson et Andrew J. Morris. « Ab initio Structure Prediction Methods for Battery Materials : A review of recent computational efforts to predict the atomic level structure and bonding in materials for rechargeable batteries ». Johnson Matthey Technology Review 64, n^o 2 (1 avril 2020) : 103–18. http://dx.doi.org/10.1595/205651320x15742491027978.

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Portable electronic devices, electric vehicles and stationary energy storage applications, which encourage carbon-neutral energy alternatives, are driving demand for batteries that have concurrently higher energy densities, faster charging rates, safer operation and lower prices. These demands can no longer be met by incrementally improving existing technologies but require the discovery of new materials with exceptional properties. Experimental materials discovery is both expensive and time consuming: before the efficacy of a new battery material can be assessed, its synthesis and stability must be well-understood. Computational materials modelling can expedite this process by predicting novel materials, both in stand-alone theoretical calculations and in tandem with experiments. In this review, we describe a materials discovery framework based on density functional theory (DFT) to predict the properties of electrode and solid-electrolyte materials and validate these predictions experimentally. First, we discuss crystal structure prediction using the Ab initio random structure searching (AIRSS) method. Next, we describe how DFT results allow us to predict which phases form during electrode cycling, as well as the electrode voltage profile and maximum theoretical capacity. We go on to explain how DFT can be used to simulate experimentally measurable properties such as nuclear magnetic resonance (NMR) spectra and ionic conductivities. We illustrate the described workflow with multiple experimentally validated examples: materials for lithium-ion and sodium-ion anodes and lithium-ion solid electrolytes. These examples highlight the power of combining computation with experiment to advance battery materials research.

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Xu, Chao, et Dong Chen. « Electronic Structures of the High-Pressure hcp and bcc Phases of Al : A Computer Aided Design and Simulation ». Applied Mechanics and Materials 556-562 (mai 2014) : 523–26. http://dx.doi.org/10.4028/www.scientific.net/amm.556-562.523.

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Thestate-of-the-artplane-wave methods combined with ultra-soft pseudo-potentials were employed to study the crystal and electronic structures (density of state, band structure) of aluminum in its hcp and bcc structures. In our computation we used the PBE functional, which predicts lattice constants very close to the experimental data. The calculations reveal that the whole valence band of Al is dominated by the 3s and 3p states while the conduction band is mainly contributed by the 3p band. The band structure shows that bcc-Al has a 0eV gap, which reflects its metallic character. The dispersion curves near the valence band maximum and conduction band minimum are quite flat. Generally speaking, our work is an attempt to study the high pressure electronic structures of Al, which needs to be verified by experiments.

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Quintas-Sánchez, Ernesto, et Richard Dawes. « Spectroscopy and Scattering Studies Using Interpolated Ab Initio Potentials ». Annual Review of Physical Chemistry 72, n^o 1 (20 avril 2021) : 399–421. http://dx.doi.org/10.1146/annurev-physchem-090519-051837.

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The Born–Oppenheimer potential energy surface (PES) has come a long way since its introduction in the 1920s, both conceptually and in predictive power for practical applications. Nevertheless, nearly 100 years later—despite astonishing advances in computational power—the state-of-the-art first-principles prediction of observables related to spectroscopy and scattering dynamics is surprisingly limited. For example, the water dimer, (H2O)2, with only six nuclei and 20 electrons, still presents a formidable challenge for full-dimensional variational calculations of bound states and is considered out of reach for rigorous scattering calculations. The extremely poor scaling of the most rigorous quantum methods is fundamental; however, recent progress in development of approximate methodologies has opened the door to fairly routine high-quality predictions, unthinkable 20 years ago. In this review, in relation to the workflow of spectroscopy and/or scattering studies, we summarize progress and challenges in the component areas of electronic structure calculations, PES fitting, and quantum dynamical calculations.

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Dahanayake, Jayangika N., Chandana Kasireddy, Jonathan M. Ellis, Derek Hildebrandt, Olivia A. Hull, Joseph P. Karnes, Dylan Morlan et Katie R. Mitchell-Koch. « Evaluating electronic structure methods for accurate calculation of 19 F chemical shifts in fluorinated amino acids ». Journal of Computational Chemistry 38, n^o 30 (21 août 2017) : 2605–17. http://dx.doi.org/10.1002/jcc.24919.

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Grant, Ian, et Harry Quiney. « GRASP : The Future ? » Atoms 10, n^o 4 (2 octobre 2022) : 108. http://dx.doi.org/10.3390/atoms10040108.

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The theoretical foundations of relativistic electronic structure theory within quantum electrodynamics (QED) and the computational basis of the atomic structure code GRASP are briefly surveyed. A class of four-component basis set is introduced, which we denote the CKG-spinor set, that enforces the charge-conjugation symmetry of the Dirac equation. This formalism has been implemented using the Gaussian function technology that is routinely used in computational quantum chemistry, including in our relativistic molecular structure code, BERTHA. We demonstrate that, unlike the kinetically matched two-component basis sets that are widely employed in relativistic quantum chemistry, the CKG-spinor basis is able to reproduce the well-known eigenvalue spectrum of point-nuclear hydrogenic systems to high accuracy for all atomic symmetry types. Calculations are reported of third- and higher-order vacuum polarization effects in hydrogenic systems using the CKG-spinor set. These results reveal that Gaussian basis set expansions are able to calculate accurately these QED effects without recourse to the apparatus of regularization and in agreement with existing methods. An approach to the evaluation of the electron self-energy is outlined that extends our earlier work using partial-wave expansions in QED. Combined with the treatment of vacuum polarization effects described in this article, these basis set methods suggest the development of a comprehensive ab initio approach to the calculation of radiative and QED effects in future versions of the GRASP code.

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Tanuma, Yuri, Toru Maekawa et Chris Ewels. « Methodological Investigation for Hydrogen Addition to Small Cage Carbon Fullerenes ». Crystals 11, n^o 11 (1 novembre 2021) : 1334. http://dx.doi.org/10.3390/cryst11111334.

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Hydrogenated small fullerenes (Cn, n < 60) are of interest as potential astrochemical species, and as intermediates in hydrogen-catalysed fullerene growth. However, the computational identification of key stable species is difficult due to the vast configurationally space of structures. In this study, we explored routes to predict stable hydrogenated small fullerenes. We showed that neither local fullerene geometry nor local electronic structure analysis was able to correctly predict subsequent low-energy hydrogenation sites, and sequential stable addition searches also sometimes failed to identify most stable hydrogenated fullerene isomers. Of the empirical and semi-empirical methods tested, GFN2-xTB consistently gave highly accurate energy correlations (r > 0.99) to full DFT-LDA calculations at a fraction of the computational cost. This allowed identification of the most stable hydrogenated fullerenes up to 4H for four fullerenes, namely two isomers of C28 and C40, via “brute force” systematic testing of all symmetry-inequivalent combinations. The approach shows promise for wider systematic studies of smaller hydrogenated fullerenes.

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SRIWICHITKAMOL, KRIENGSAK, SONGWUT SURAMITR, POTJAMAN POOLMEE et SUPA HANNONGBUA. « STRUCTURES, ABSORPTION SPECTRA, AND ELECTRONIC PROPERTIES OF POLYFLUORENE AND ITS DERIVATIVES : A THEORETICAL STUDY ». Journal of Theoretical and Computational Chemistry 05, n^o 03 (septembre 2006) : 595–608. http://dx.doi.org/10.1142/s0219633606002520.

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The structural and energetic properties of polyfluorene and its derivatives were investigated, using quantum chemical calculations. Conformational analysis of bifluorene was performed by using ab initio (HF/6-31G* and MP2/6-31G*) and density functional theory (B3LYP/6-31G*) calculations. The results showed that the local energy minimum of bifluorene lies between the coplanar and perpendicular conformation, and the B3LYP/6-31G* calculations led to the overestimation of the stability of the planar pi systems. The HOMO-LUMO energy differences of fluorene oligomers and its derivatives — 9,9-dihexylfluorene (DHPF), 9,9-dioctylfluorene (PFO), and bis(2-ethylhexyl)fluorene (BEHPF) — were calculated at the B3LYP/6-31G* level. Energy gaps and effective conjugation lengths of the corresponding polymers were obtained by extrapolating HOMO-LUMO energy differences and the lowest excitation energies to infinite chain length. The lowest excitation energies and the maximum absorption wavelength of polyfluorene were also performed, employing the time-dependent density functional theory (TDDFT) and ZINDO methods. The extrapolations, based on TDDFT and ZINDO calculations, agree well with experimental results. These theoretical methods can be useful for the design of new polymeric structures with a reducing energy gap.

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