Validity of the multipole results for first-order molecule-molecule interaction energies.

Abstract

The HF-HF, LiH-LiH and N₂-N₂ interactions, together with previous work on the H₂-H₂ interaction, are used as models for discussing how charge overlap effects in the first-order Coulomb energy vary as a function of orientation and intermolecular separation as the nature of the interacting species changes. For interactions such as H₂-H₂ and HF-HF, where the interacting molecules have positive even permanent multipole moments, charge overlap effects are relatively small and the usual R ^-1 multipole results for the interaction energy between the permanent moments of the interacting molecules give a remarkably good representation of the first-order Coulomb energy until R becomes quite small. For interactions such as LiH-LiH and N₂-N₂, where the interacting molecules have negative even permanent moments, charge overlap effects are large as a relative function of R and the multipolar results yield a poor representation of the interaction energy even for moderately large values of R. Some of the problems and errors that can arise in the evaluation or the interpretation of the bulk properties of matter, if the first-order R ^-1 multipolar energies are used to represent the anisotropic part of the intermolecular potential, are discussed using N₂-N₂ as an example.