The forces between polyatomic molecules. II. Short-range repulsive forces

Abstract
The Hellmann-Feynman theorem is used to examine the qualitative behaviour of short-range repulsive forces between closed-shell polyatomic molecules whose electron clouds overlap slightly. It is found that the most important contribution to the repulsive forces arises from a reduction in combined electron density in the region of overlap. Using this result it is possible to show that for two molecules whose electron density is best described by localized orbitals, and for which the orbitals of one molecule do not overlap the orbitals of the other in more than one localized interatomic region, the forces are locally additive over pairs of atoms, one in each of the interacting molecules. However, those molecules which are adequately described by orbitals which extend over the molecular framework and which can overlap opposite orbitals in more than one interatomic region, will not interact in a locally additive way.