The calculation of intermolecular potential energy surfaces I. Basic theory

Abstract

The potential energy of interaction between two molecules is expanded in powers of electron interchange. The spin-coupled formalism, where space and spin coordinates are explicitly separated, is used and this leads to compact and systematic expressions for the various terms in the expansion. A comprehensive diagram technique is developed with the aid of which any term in the expansion can be obtained by inspection . The range of application of the theory is discussed and it is shown that this approach is particularly suitable for interactions between non-bonded systems whose overlap with each other is always small. For bonded systems, the method is suitable for interactions in the long and intermediate ranges.