MANY-BODY PERTURBATION-THEORY OF ELECTROSTATIC INTERACTIONS BETWEEN MOLECULES - COMPARISON WITH FULL CONFIGURATION-INTERACTION FOR 4-ELECTRON DIMERS

Many-body perturbation theory for a direct calculation of the electros tatic interaction energy is developed. Since no multipole expansion is used, the obtained electrostatic energy includes the short-range cont ributions resulting from the overlap (penetration) of monomers' charge distributions. The influence of intramonomer electronic correlation i s accounted for by the perturbation expansion in terms of the Moller-P lesset type fluctuation potentials for the interacting molecules. Two types of expansions are introduced: one based on the standard Moller-P lesset expansion of the electron density, and the second accounting fo r the perturbation induced modifications of the monomer's Fock operato rs, i.e., for the so-called response or orbital relaxation effects. Ex plicit orbital expressions for the terms through the fourth order in t he intramonomer fluctuation potentials are derived. In this way the le ading three-particle correlation contribution to the electrostatic ene rgy is taken into account. Numerical results of perturbative calculati ons through the fourth order for the He-2, (H-2)2, and He-H-2 systems are presented and compared with the complete configuration interaction results obtained using the same basis sets. It is found that the conv ergence of the many-body perturbation expansion of the electrostatic e nergy is fast. The sum of corrections through the fourth order reprodu ces to within few percent the correlation part of the electrostatic en ergy computed with the full configuration interaction wave function.