Interaction of chemical bonds. V. Perturbative corrections to geminal-typewave functions

Antisymmetrized products of strongly orthogonal geminals (APSG) and approxi mations thereof, provide a variational and size-consistent wave function bu t do not account for a sufficient amount of dynamical electron correlation. The latter effects can only be calculated by considering intergeminal inte ractions. In this study we present a general scheme for computing delocaliz ation effects involving simultaneous interactions of two, three, and four g eminals. We also describe a simple perturbative procedure, the APSG-MP2 met hod, by which APSG-type wave functions and the associated energies can be i mproved at a very low computational expense. (C) 2000 John Wiley & Sons, In c.