Calcul ab initio des fréquences vibrationnelles anharmoniques de Hobo

Les energies, gradients et hessiens ont été évalués pour 89 structures géométriques distordues de la molécule HOBO à partir de calculs ab-initio de type MP2 et MP4-SDQT. Les surfaces d'energie potentielle correspondantes ont été ajustées au sens des moindres carrés pour calculer les constantes de force cubiques et quartiques permettant d'obtenir les fréquences fondamentales anharmoniques au moyen d'une méthode de variation-perturbation. Les résultats théoriques recoupent avec une grande fiabilité les résultats expérimentaux les plus surs et permettent l'attribution complète de toutes les bandes fondamentales non attribuées pour HOBO et DOBO

Ab initio Cl determination of electric moments and frequency dependent polarizabilities and hyperpolarizabilities for CO

Frequency-dependent electric dipole polarizabilities of CO were computed using a time-dependent gauge invariant (TDGI) method, accounting for a large part of the correlation effects in response properties. The results compare well with both experimental and other theoretical calculations. Reported results also include the Ce dispersion coefficients and the first frequency dependent hyperpolarizability

Choix de la jauge dans le calcul quantique de propriétés électromagnétiques des molécules

Le calcul quantique de propriétés électroniques comme les susceptibilités électriques ou magnétiques nécessite de définir une jauge pour décrire le champ électromagnétique. Dans les ”conditions habituelles” de calcul, les résultats sont dépendants de jauge et le choix de celle-ci devient important. Ce problème est bien connu dans le cas du calcul des susceptibilités magnétiques [1-9], mais peut-être moins dans le cas du calcul des polarisabilités. Nous montrerons l’effet d’un changement de jauge sur le calcul des polarisabilités dynamiques de l’atome d’hydrogène

DFT modelling of the infrared spectra for isolated and aqueous forms of adenine

International audienceThis theoretical study provides the anharmonic vibrational spectra of isolated and aqueous forms of adenine. The calculations were performed at the DFT B3LYP/6-31+G(d,p) level of theory using two different ways for the treatment of the anharmonicity: time-independent (VPT2) and time-dependent (molecular dynamics) methods. The wavenumbers obtained from both treatments are compared to experimental data for the two forms. Globally, the VPT2 method appears slightly better to describe the mid-IR spectra in the fingerprint region between 600 and 1700 cm−1 for the isolated form, the two approaches leading similar results for the aqueous adenine. Finally, the structural model of solvation used for aqueous adenine that combines an explicit solvent model with a polarizable continuum model gives, as in the case of cytosine, thymine and guanine, previously studied, excellent results. © 2017, Springer-Verlag Berlin Heidelberg

Hirshfeld-based atomic population analysis of the B, N doping effect in zigzag graphene nanoribbons: π electron density as requirement to follow the B, N doping guidelines

International audienceIn this work, we present an atomic population study within the Fractional Occupation Hirshfeld-I (FOHI) scheme applied to several pristine, boron-, nitrogen- and B, N-doped non-periodic zigzag graphene nanoribbons (always using a double carbon replacement). To accomplish this task, we have considered the singlet–triplet energy gap as a criterion to check the most reliable electron density. B2PLYPD double-hybrid functional provides the most accurate relative energies in comparison with the other methods, but similar atomic populations are obtained in most cases. Moreover, in spite of the observed different behavior concerning the population of B, N dopants and their corresponding p- and n-type doping effects, the FOHI atomic populations are in excellent agreement with the widely accepted electronegative scale. Nevertheless, we propose to employ a more appropriate electron partitioning strategy taking into account the contribution of π-symmetric orbitals. It provides the expected population results according to doping guidelines. In any case, both kinds of populations describe in a similar way the mesomeric effects and the edge variations after replacing two carbons by either two boron or nitrogen atoms. On the contrary, the populations point out a different behavior when the systems are doped with one boron and one nitrogen simultaneously

An Ab-initio Hartree-fock Study of Electronic and Structural-properties of Mgh2

A periodic ab initio Hartree-Fock method has been used to evaluate a number of electronic and structural properties of MgH2. The calculated quantities are the equilibrium lattice parameters, the cohesive energy, the elastic constants, the energy-band structure, the density of states, the electronic charge distribution and the Compton profiles. For the two former properties correlation effects are taken into account and improve the agreement with the experimental data.The strongly but not fully ionic character of MgH2 is confirmed by the present study

Vibrational spectra of difluorosilane from a hybrid ab-initio and DFT quartic force field

cited By 12International audienceFundamental frequencies of SiH2F2 and their isotopic shifts are computed using a hybrid quartic potential in which CCSD(T)/cc-pVTZ equilibrium values and harmonic wave-numbers are coupled to B3LYP/6-31+G** anharmonic terms in the framework of a variational treatment working in several spectral ranges. The mean-square deviation between our computed values and the most reliable experimental data are only 0.8% (SiH2F2) and 0.6% (SiD2F2). Calculation of both overtones and combination bands in the medium infrared region suggests a new assignment of some observed bands. © 2005 Elsevier B.V. All rights reserved

Calcul ab initio du champ de force quartique et du spectre vibrationnel enharmonique de la molécule de méthylène-imine H

Les énergies gradients et hessiens ont été évalués pour 370 structures géométriques distordues de la molécule H2CNH à partir de calculs de type MP2/6-31 IG**. La surface d'énergie potentielle correspondante a été ajustée au sens des moindres carrés pour calculer les constantes de force cubiques et quartiques. Ces constantes ont été utilisées pour obtenir les fréquences anharmoniques au moyen de méthodes de type variationnel, perturbationnel ou de variation-perturbation qui seront comparées. Les résultats théoriques recoupent avec une grande fiabilité les résultats expérimentaux les plus sûrs et permettent l'attribution de toutes les bandes fondamentales de H2CNH et de huit de ses isomères