B. Levy

Valence-bond calculations with polarized atomic orbitals

Abstract A new method for computing polarized atomic orbitals (PAOs) is described: this method leads to very easy calculations. The space of the resulting PAOs is close to that of MC SCF MOs. Using these PAOs in the frame of a VB calculation has led to the same level of accuracy as the comparable MC SCF calculation for the dissociation energies and the lowest electronic transition energies of H2, H3 and N2.

Robust and Efficient Constrained DFT Molecular Dynamics Approach for Biochemical Modeling.

Constrained density functional theory (cDFT) is a powerful tool to investigate the dynamics of the electrons accompanying various physical-chemical processes. In this article we present our recent progresses in the implementation of the method in the parallelized version of the DFT program deMon2k. We take advantage of the possibility to express atomic densities in terms of linear combination of Hermite Gaussian functions to improve the computation of the cDFT integration weights within the Hirshfeld and Voronoi deformation density electronic population approaches. The efficiency of the method is illustrated on the computation of the average electronic coupling for an electron transfer (ET) through a glycine polypeptide of increasing length. The sampling is based on cDFT and hybrid cDFT/molecular mechanics molecular dynamics simulations. We also report the first estimations of quantum decoherence times from cDFT-based simulations for an ET reaction.

Calculation of the nuclear magnetic shielding of 29Si, 31P, 33S and 35Cl using a pseudo-potential method

The nuclear magnetic shielding of 29Si, 31P, 33S and 35Cl in SiH4, PH3, SH2 and ClH has been computed. The inner shells of the heavy atom are not introduced in the calculation for the molecule. Their effect on the valence shell is represented by a pseudo-potential. The agreement with available theoretical and experimental values is satisfactory.

Orbitales moléculaires S.C.F. localisées. Etude de barrières d'inversion

The nitrogen inversion barriers in aziridine, oxaziridine and methylenimine and the carbanion inversion barrier in the vinyl anion have been studied in terms of localized SCF molecular orbitals. The changes in the interactions between localized orbitals (bonds, inner shells, lone pairs) as inversion proceeds are analyzed by distinguishing a “primary” effect limited to the NH (or CH⊖) site, and a “secondary” effect arising from the remainder of the molecule.Both factors play a role in the determination of the barrier height. All pairwise interactions are significant. The energetical origin of the barriers cannot be localized at a given molecular site (e.g. NH) but is delocalized over the whole molecule.RésuméLes barrières dinversion de lazote dans laziridine, loxaziridine et la méthylène imine et la barrière dinversion du carbanion dans lanion vinyle ont été étudiées à laide dorbitales moléculaires S.C.F. localisées. La variation des interactions entre orbitales localisées (liaisons, couches internes, doublets libres) lors de linversion permet de distinguer un effet « primaire » limité au site NH (ou CH⊖), et un effet « secondaire » provenant du reste de la molécule.Les deux facteurs interviennent dans la détermination de la barrière dinversion; toutes les interactions deux à deux sont significatives. Lorigine énergétique des barrières ne peut être localisée en un site donné (par ex. NH) mais est délocalisée sur toute la molécule.ZusammenfassungDie Inversionsbarriere des Stickstoffs in Aziridin, Oxaziridin und Methylenimin sowie des Carbanions im Vinylanion wurde mit Hilfe lokalisierter SCF-MOs untersucht. Die Änderungen der Wechselwirkungen zwischen lokalisierten Orbitalen (Bindungen, inneren Schalen, einsamen Elektronenpaaren) im Verlauf der Inversion werden analysiert; dabei wird zwischen einem „primären” Effekt auf der Seite der NH- (oder CH⊖)-Gruppe und einem „sekundären“ Effekt auf seiten des Molekülrests unterschieden.Beide Faktoren spielen eine Rolle für die Höhe der Barriere. Alle paarweisen Wechselwirkungen sind signifikant. Der Energiebeitrag zur Barriere kann nicht auf einen bestimmten Molekülteil (z.B. NH) beschränkt werden, sondern ist auf das ganze Molekül verteilt.

Ab initio determination of vibrational band intensities

We have determined the vibrational band intensities of the 1 A 2←1 A 1 transition of formaldehyde by means of an ab initio calculation. The values obtained fit correctly the experimental evaluations. It is also found that confining the Herzberg-Teller type expansion to the first non-zero term is a poor approximation to the exact value.

Localization and Relocalization in Orbital Theories

In the first part of this review, the role played by the localization-delocalization alternative and by the a posteriori relocalization techniques in the development of the molecular orbital theory is discussed. Illustrative examples are given, the emphasis being laid on some difficulties usually encountered when applying standard localization techniques. In the second part, an unified treatment of the a priori localizing methods suggested on several occasions is presented.

SCF CI calculations of the K-shell ionization potential of carbon in methane and in the fluoromethanes

Abstract The vertical ionization potentials of the K-shell of carbon have been calculated in CH 4 , CH 3 F, CH 2 F 2 , CHF 3 and CF 4 . The ab initio SCF CI approach has been used for the first three compounds with an attempt to rationalize the choice of the atomic orbital basis sets. The ionization potentials of CHF 3 and CF 4 have been extrapolated from the proceeding results. The effect of the CI cannot be neglected in either of these compounds, but it can in CH 4 .

Potential energy surface of H2O+: linear approach

An SCF+CI calculation of the potential energy surface of H2O+ in Cinfinity v symmetry (linear approach) is reported. No energy barrier has been found for the chemical reaction O++H2 to OH++H. Instead, the energy of the intermediate species (O...H...H)+ is found to be 12 kcal/mole below the energy of the input products. This leads to the conclusions that the reaction can actually occur at low temperatures (interstellar clouds) and that the complete potential energy surface is worth computing.

Transfer of electronic excitation: The Arh system

Abstract A theoretical analysis of the quenching of Ar* in its two metastable states (3P2 and 3P0) by H is presented. It is shown that a simplified treatment of the electronic structure permits a discussion of the dynamics of that type of excitation transfer in terms of the relative position of a crossing (RC) of potential curves and of the region (RD) of decoupling of atomic states by the molecular field. In the present case we explain why the 3P2 state is more reactive than the 3P0 state.

Analysis of NMR and EPR Coupling Constants Using Localized Orbitals

The EPR and NMR couplings are discussed in terms of localized molecular orbitals using the double perturbation theory. In the first part, a general study of the through-space through-bond dilemma concerning the mechanism of EPR and NMR coupling between non-bonded sites is presented. In the second part, the terms useful for explaining the NMR coupling between protons at different distances (geminal, vicinal and long-range coupling constants) are identified. A special attention is given to the geometrical and substituent effects on the 3 J HH’ coupling constant in etbylen. In the last part, a discussion of the so-called W-rule concerning the EPR coupling of a proton in γ position of the radical center is made. In parts II and III, illustrative examples based on results coming from semi-empirical and ab-initio calculations are given.