Perturbation Theory for Intermolecular Interactions in the Wave-Operator Formalism

Abstract

A Schrödinger‐type perturbation theory for intermolecular or interatomic interactions is developed in the wave‐operator formalism. To include all exchange effects, wavefunctions are used whose symmetry with respect to permutations of both electronic and nuclear coordinates can be prescribed arbitrarily. This necessitates a modification of the usual perturbation theory. The interaction energy is obtained as a series in ascending powers of the interaction operator. It is proved that every term in this series is real and that the terms of even order are negative definite for perturbation of the ground state. It is verified that up to and including third order the results of this theory, if they are restricted to electron exchange only, agree exactly with those of the Eisenschitz—London theory.