Perturbation Theory for Exchange Interactions between Atoms and Molecules. I

Abstract

A perturbation theory has been developed for interatomic or intermolecular forces with exchange. The method requires the use of label-free operators in a representation of nonorthogonal, antisymmetrized states. The introduction of this representation shows that the explicit construction of a complete orthogonalized set is unnecessary. The antisymmetry of the exact wave function is preserved to all orders, and hence exchange is treated exactly.