Interaction of chemical bonds. II. Ab initio theory for overlap, delocalization, and dispersion interactions.

Abstract

A zeroth-order wave function is constructed as an antisymmetrized product of two-electron group wave functions (geminals) expanded in disjunct but overlapping subspaces of basis orbitals. The geminals are obtained as exact solutions of the two-electron Schr\"odinger equations within the corresponding local basis sets, and thus give a fully correlated description of the two-electron chemical bonds coupled by inductive (Coulomb and exchange) effects, the latter being taken into account by an appropriate effective core operator. A second-quantized formulation [P. R. Surj\'an, Phys. Rev. A 30, 43 (1984), Part I] is applied where the wave functions of the individual bonds are represented by appropriate composite particle creation operators. Individual chemical bonds thus correspond to Bose quasiparticles composed of two electrons. Second-order perturbation theory is used for calculating interbond delocalization and dispersion effects. The treatment is based on a biorthogonal formulation [I. Mayer, Int. J. Quant. Chem. 23, 341 (1983); Ph. W. Payne, J. Chem. Phys. 77, 5630 (1982)] which makes the handling of interbond overlap very effective, and represents essentially a method of moments in the perturbation theory.