Interaction of chemical bonds.: V.: Perturbative corrections to geminal-type wave functions

Antisymmetrized products of strongly orthogonal geminals (APSG) and approximations thereof, provide a variational and size-consistent wave function but do not account for a sufficient amount of dynamical electron correlation. The latter effects can only be calculated by considering intergeminal interactions. In this study we present a general scheme for computing delocalization effects involving simultaneous interactions of two, three, and four geminals. We also describe a simple perturbative procedure, the APSG-MP2 method, by which APSG-type wave functions and the associated energies can be improved at a very low computational expense. (C) 2000 John Wiley & Sons, Inc.