Extensions of the siesta dft code for simulation of molecules

We describe extensions to the siesta density functional theory (dft) code [30], for the simulation of isolated molecules and their absorption spectra. The extensions allow for: - Use of a multi-grid solver for the Poisson equation on a finite dft mesh. Non-periodic, Dirichlet boundary conditions are computed by expansion of the electric multipoles over spherical harmonics. - Truncation of a molecular system by the method of design atom pseudo- potentials of Xiao and Zhang[32]. - Electrostatic potential fitting to determine effective atomic charges. - Derivation of electronic absorption transition energies and oscillator stren- gths from the raw spectra produced by a recently described, order O(N3), time-dependent dft code[21]. The code is furthermore integrated within siesta as a post-processing option

Réactivité de surface d'oxydes lamellaires, matériaux d'électrode positive dans des accumulateurs au lithium approches expérimentale et théorique

L'objectif de ce travail est de contribuer, par le biais d'approches expérimentales (XPS/chimisorption de sondes gazeuses) et théoriques (approches type DFT), à une meilleure compréhension fondamentale de la réactivité de surface de matériaux d'électrode positive et plus spécifiquement d'oxydes lamellaires lithiés LiMO2. La réactivité de surface du matériau LiCoO2 et l influence d une substitution de l atome de cobalt par l atome d aluminium a été étudiée pour avancer dans la compréhension de l effet bénéfique des coatings à base d alumine. L étude expérimentale a été centrée sur l adsorption de SO2 et montre que la présence d atomes d aluminium induit une diminution de réactivité : elle génère des espèces de type sulfite, différentes des sulfates seuls identifiés pour LiCoO2. La modélisation de cette adsorption permet la mise en évidence de deux modes d adsorption distincts : passage d un contrôle rédox pour LiCoO2 à un contrôle acido-basique pour des matériaux contenant des atomes d aluminium (LiAlO2), thermodynamiquement moins favorable. L influence de la nature du métal de transition a été analysée sur la base d études théoriques également menées sur LiMnO2 et LiNiO2. La modélisation de l adsorption de SO2 sur les surfaces des matériaux LiMO2 révèle la présence des deux processus de chimisorption (formation de sulfites et sulfates), avec mise en évidence du rôle important des cations de sous-surface dans les processus de réduction conduisant aux sulfates. Cette étude théorique a permis d interpréter les résultats expérimentaux obtenus pour Li(Ni1/3Mn1/3Co1/3)O2, l identification de sulfites résulterait de la présence des ions Ni2+ alors que celle de sulfates serait liée à la présence des ions Mn4+ et Co3+.This work is devoted to a better fundamental understanding of the surface reactivity of positive electrode materials, and specifically lamellar lithiated oxides LiMO2 through experimental (XPS/chemisorption of gaseous probes) and theoretical (DFT calculations) approaches. The beneficial effect of aluminum-based coatings on electrochemical performances is well known but the exact mechanisms are not totally understood. A detailed study of the surface reactivity of LiCoO2 and of the influence of Al/Co substitution is carried out. The experimental approach is focused on SO2 adsorption and shows that Al/Co substitution induces a decrease of the surface reactivity and a change in the nature of adsorbed species (identification of sulfite species whereas only sulfate species are characterized for LiCoO2). Theoretical calculations highlighs two different adsorption modes controlled by redox properties for LiCoO2 and by acid-base properties for -LiAlO2 (less energetically favorable). The theoretical study is extended to LiMnO2 and LiNiO2 in order to analyse the influence of the nature of the transition metal atom. The modelization of SO2 adsorption reveals two chemisorption processes (sulfite and sulfate formation), and highligths the key role of subsurface cations in the reduction process which leads to sulfates. Thus theoretical approach allows on interpretation of the experimental data obtained for Li(Ni1/3Mn1/3Co1/3)O2. The identification of sulfites may be explained by the presence of Ni2+ ions whereas sulfate species may result from the presence Mn4+ and Co3+ ions.PAU-BU Sciences (644452103) / SudocSudocFranceF

Nitrile ylides: allenic and propargylic structures from pyrazinylnitrenes. Experimental and theoretical characterization

Matrix photolysis of 2-pyrazinyl azides/tetrazolo[1,5-a]pyrazines generates nitrile ylides 15 via pyrazinylnitrenes 13 and triazacycloheptatetraenes 14. The nitrile ylides 15 are characterized by IR spectroscopy in conjunction with harmonic and anharmonic vibrational frequency calculations. The nitrile ylides exist in the matrices in the Z,Z-conformations in which they are born. Substitution on the nitrile carbon of nitrile ylides has a profound effect on their structure. Even different conformers of the same molecule can have differences up to 200 cm in the IR absorptions of the ylide moieties. Nitrile ylides 15a and 15b (R = H or Cl, R' = H) have allenic structures (15 Allenic). Nitrile ylide 15c (R = R′ = CH) has a distinctly propargylic structure (15 Propargylic) in the experimentally observed Z,Z-conformation

Iminopropadienones RN=C=C=C=O and bisiminopropadienes RN=C=C=C=NR: Matrix infrared spectra and anharmonic frequency calculations

Methyliminopropadienone MeN=C=C=C=O 1a was generated by flash vacuum thermolysis from four different precursors and isolated in solid argon. The matrix-isolation infrared spectrum is dominated by unusually strong anharmonic effects resulting in complex fine structure of the absorptions due to the NCCCO moiety in the 2200 cm-1 region. Doubling and tripling of the corresponding absorption bands are observed for phenyliminopropadienone PhN=C=C=C=O 1b and bis(phenylimino)propadiene PhN=C=C=C=NPh 9, respectively. Anharmonic vibrational frequency calculations allow the identification of a number of overtones and combination bands as the cause of the splittings for each molecule. This method constitutes an important tool for the characterization of reactive intermediates and unusual molecules by matrix-isolation infrared spectroscopy

Using computed infrared intensities for the reduction of vibrational configuration interaction bases

International audienceThe Adaptive Vibrational Configuration Interaction (A-VCI) algorithm is an iterative process that computes the anharmonic spectrum of a molecule using nested bases to discretize the Hamiltonian operator. For large molecular systems, the size of the discretization space and the computation time quickly become prohibitive. It is therefore necessary to develop new methods to further limit the number of basis functions. Most of the time, the interpretation of an experimental infrared spectrum does not require the calculation of all eigenvalues but only those corresponding to vibrational states with significant intensity. In this paper, a technique that uses infrared intensities is introduced to select a subset of eigenvalues to be precisely calculated. Thus, we build smaller nested bases and reduce both the memory footprint and the computational time. We validate the advantages of this new approach on a well-studied 7-atom molecular system (C2H4O), and we apply it on a larger 10-atom molecule (C4H4N2)

Using computed infrared intensities for fast computation of vibrational spectra

The Adaptive Vibrational Configuration Interaction (A-VCI) algorithm is an iterative pro-cess able to compute the spectrum of an Hamiltonian operator, using a discretization basisas small as possible. In this work, we show how this algorithm can handle more sophis-ticated operators, which ro-vibrational Coriolis coupling terms. In order to overcome theincrease of computing and storage resources needed due to this enrichment, the InfraRed(IR) intensities are computed and used as a criterion to select only the eigenstates corre-sponding to IR active vibrational states. The benefits of this new approach are presentedfor a few well studied molecular systems (H2O, H2CO, CH2NH, CH3CN, C2H4O), and itis ultimately applied to a 10-atom molecule (C4H4N2

A-VCI: une méthode flexible pour calculer rapidement des spectres vibrationnels

The Adaptive Vibrational Configuration Interaction (A-VCI) algorithm has been introduced as a new method to efficiently reduce the dimension of the set of basis functions used in a Vibrational Configuration Interaction (VCI) process. It is based on the construction of nested basis for the discretization of the Hamiltonian operator according to a theoretical criterion that ensures the convergence of the method. The purpose of this paper is to study the properties and outline the performance details of the main steps of the algorithm. New parameters have been incorporated to increase flexibility, and their influence have been thoroughly investigated. The robustness and reliability of the method are demonstrated for the computation of the vibrational spectrum up to $3000$ cm$^{-1}$ of a widely studied $6$-atom molecule (acetonitrile). Our results are compared to the most accurate up to date computation, and we also give a new reference calculation for future work on this system. The algorithm has also been applied to a more challenging $7$-atom molecule (ethylene oxide). The computed spectrum up to $3200$ cm$^{-1}$ is the most accurate computation that exists today on such a system.L'algorithme adaptatif d'interaction de configuration vibrationnelle (A-VCI) a été introduit comme une nouvelle méthode pour réduire efficacement la dimension de l'ensemble des fonctions de base utilisées dans un processus d'interaction de configuration vibrationnelle (VCI). Il est basé sur la construction de bases emboîtées pour la discrétisation de l'opérateur Hamiltonien selon un critère théorique qui assure la convergence de la méthode. Cet article présente les propriétés de la méthode et décrit les détails des principales étapes de l'algorithme. De nouveaux paramètres sont introduits pour accroître les potentialités de la méthode et leurs influences sont étudiées. La robustesse et la fiabilité de la méthode sont démontrées pour le calcul du spectre vibrationnel jusqu'à $3000$ cm $^ {- 1}$ d'une molécule à 6 atomes (acétonitrile). Nos résultats sont comparés au calcul le plus précis à jour, et nous donnons également un nouveau calcul de référence pour les travaux futurs sur ce système. L'algorithme a également été appliqué à un système plus difficile l'oxyde d'éthylène qui comporte 7 atomes. Le spectre calculé jusqu'à $3200$ cm $^{- 1}$ est le calcul le plus précis qui existe aujourd'hui sur un tel système

Efficient basis selection for the computation of vibrational spectrum

International audienceWe propose here an efficient method to define an approximation subspace to compute the first vibrational frequencies of the molecular Hamiltonian which are those of interest in the experimental results

Optimisation of the variational method for vibrational Hamiltonian eigenvalues computation

International audienceWe propose here an efficient method to define a representative approximation subspace to compute the first eigenvalues of the vibrational Hamiltonian which are those of interest in the experimental results

Adaptive vibrational configuration interaction (A-VCI): a posteriori error estimation to efficiently compute anharmonic IR spectra

International audienceA new variational algorithm called adaptive vibrational configuration interaction (A-VCI) intended for the resolution of the vibrational Schrödinger equation was developed. The main advantage of this approach is to efficiently reduce the dimension of the active space generated into the configuration interaction (CI) process. Here, we assume that the Hamiltonian writes as a sum of products of operators. This adaptive algorithm was developed with the use of three correlated conditions i.e. a suitable starting space ; a criterion for convergence, and a procedure to expand the approximate space. The velocity of the algorithm was increased with the use of a posteriori error estimator (residue) to select the most relevant direction to increase the space. Two examples have been selected for benchmark. In the case of H 2 CO, we mainly study the performance of A-VCI algorithm: comparison with the variation-perturbation method, choice of the initial space, residual contributions. For CH 3 CN, we compare the A-VCI results with a computed reference spectrum using the same potential energy surface and for an active space reduced by about 90 %