P. Claverie

Theoretical studies of molecular conformation. Derivation of an additive procedure for the computation of intramolecular interaction energies. Comparison withab initio SCF computations

An additive procedure (SIBFA) is developed for the rapid computation of conformational energy variations in very large molecules. The macromolecule is built out of constitutive molecular fragments and the intramolecular energy is computed as a sum of interaction energies between the fragments. The electrostatic and the polarization components are calculated using multicenter multipole expansions of theab initio SCF electron density of the fragments. The repulsion component is obtained as a sum of bond and lone pair interactions.Tests of the procedure on a series of model compounds containing ether oxygens and pyridine-like nitrogens are reported and compared with the results of correspondingab initio SCF calculations. The resulting methodology is compatible with the simultaneous computation of intermolecular interactions.

Localized bond orbitals and the correlation problem: I. The perturbation calculation of the ground state energy

A method is proposed to go beyond the SCF result in the calculation of the ground state energies without any variational procedure. One chooses a set of reasonable bonding and antibonding orbitals localized on the chemical bonds. The bonding orbitals are used to built a fully localized determinant. The basis of excited states is built using the antihonding orbitals. One calculates the lower eigenvalue of the CI matrix in this basis by a Rayleigh-Schr6dinger expansion. The conceptual and practical advantages of the method are discussed, and the perturbation series is specified in order to satisfy the linked cluster theorem conditions and to retain the advantages of the Epstein-Nesbet partition of the Hamiltonian. Es wird eine Methode angegeben, die die SCF-Resultate in der Berechnung der Grundzustandsenergien tibertrifft, ohne die Mittel der Variationsrechnung anzuwenden. Als Basisfunktionen w~hlt man einen Satz geeigneter bindender and antibindender Orbitale, die auf den chemischen Bindungen lokalisiert sind. Die bindenden Orbitale werden zum Aufbau einer v611ig lokalisierten Determinante benutzt. Als Basisfunktionen ftir die angeregten Zust[inde benutzt man auch antibindende Orbitale. Man berechnet den tieferen Eigenwert der CI Matrix mit dieser Basis durch eine Rayleigh-Schr6dingerEntwicklung. Die begriffiichen und praktischen Vorteile dieser Methode werden diskutiert. Indem man die St~Srungsreihe so w~ihlt, dab das linked cluster-Theorem erftillt ist, bleiben die Vorteile der Epstein-Nesbet-Aufspaltung des Hamiltonoperators erhalten.

Fully localized bond orbitals and the correlation problem

Abstract Perturbation theory in conjunction with the use of fully localized bond orbitals provides a precise and fast method for calculating electronic energy of molecules. This is illustrated in the framework of semi-empirical thoeries and the physical interpretations allowed by the method are shown.

Practical improvements for the calculation of intermolecular energies

Abstract Practical improvements for the semi-empirical calculation of long-range and short-range energies between two arbitrary molecules are given. Interaction energies between molecules of organic solvents are calculated and compared with experimental vaporization energies.

Theoretical studies of molecular conformation. II: Application of the SIBFA procedure to molecules containing carbonyl and carboxylate oxygens and amide nitrogens

The SIBFA procedure (Sum of Interactions Between Fragments computedAb initio, Ref [1]) is extended to the study of the conformational behavior of representative molecules containing amide nitrogens and carbonyl and carboxylate oxygens. The molecules studied are C- and N-ethylammonium formamide, C- and N-ethanol-formamide, ethylammonium formate and ethanolformate. The cases investigated include interactions of the types which occur between functional groups in proteins or ionophores. The accuracy of the procedure, assessed by comparing the results to those of correspondingab initio SCF computations, is very satisfactory. An application of the procedure to study the conformation of the glycyl and alanyl dipeptides as a function of the backbone torsional angles φ and ψ is presented.

On the binding of water to the ammonium ion: The interplay of an improved basis set. Dispersion and zero-point energy

Abstract The binding energy of the NH 4 + -H 2 O complex computed by Hartree-Fock calculations in an extended polarized basis set, supplemented by a perturbation calculation of the dispersion contribution and inclusion of the zero-point energy, is brought into fair agreement with the enthalpy value measured in the gas phase.

Oriented interaction between naphthalene and alkane molecules: Theoretical interpretation of spectral shifts of ππ* bands of aromatic hydrocarbons in alkane solvents

Abstract Several features of the spectral shifts of ππ * bands for aromatic hydrocarbons dissolved in n -alkane solvents have been explained by the existence of preferred relative orientations between the aromatic hydrocarbon and the n -alkane molecules. Theoretical evaluation of the interaction energy between naphthalene and n -pentane or n -octane actually display two preferred orientations. Moreover, these orientations are identical with those proposed, on steric grounds, for explaining the adsorption of alkanes on graphitic basal planes as proposed by Groszek.

Use of perturbation methods for the study of configuration interaction effects

Localized SCF orbitals are obtained by using the self-energy criteria in the framework of semiempirical methods (Pariser-Parr, Pople-Segal). The delocalisation problem is discussed. These localized orbitals (both occupied and virtual) are used for the calculation of the second-order correlation energy. Qualitative and quantitative features are discussed. The use of localized orbitals allows a deep reduction of the significant terms in the correlation energy.ZusammenfassungEs werden lokalisierte Orbitale mittels des Selbstenergie-Kriteriums im Rahmen semiempirischer Methoden (Pariser-Parr, Pople-Segal) konstruiert und das Delokalisierungsproblem diskutiert. Mit diesen lokalisierten Orbitalen (besetzt und virtuell) wird die Korrelationsenergie zweiter Ordnung berechnet, was zu einer erheblichen Reduktion wichtiger Terme führt. Die Ergebnisse werden diskutiert.RésuméDans le cadre des méthodes semi-empiriques (Pariser-Parr, Pople-Segal) on obtient des orbitales localisées S.C.F. en utilisant essentiellement le critère de localisation dEdmiston et Ruedenberg. Le problème de la délocalisation est discuté. Ces orbitales localisées (occupées et virtuelles) sont utilisées pour le calcul de lénergie de corrélation au second-ordre de la théorie des perturbations. Les modifications qualitatives et quantitatives apportées par lemploi des orbitales localisées sont discutées. Elles permettent de réduire massivement le nombre de termes nécessaires à lévaluation de lénergie de corrélation.

Study of solute-solvent interactions

Abstract A microscopic model is proposed for the calculation of solute-solvent interaction energies. Calculated energies for gaseous solute-nitrobenzene systems are compared with experimental dissolution energies.

Statistical and Stochastic Aspects of the Delocalization Problem in Quantum Mechanics

The purpose of this paper is to review what it is possible nowadays to say about space-time behaviour of electrons in atoms and molecules.