Long range intermolecular forces in triatomic systems: connecting the atom–diatom and atom–atom–atom representations

Abstract

The long-range forces that act between three atoms are analysed in both atom–diatom and atom–atom–atom representations. Expressions for atom–diatom dispersion coefficients are obtained in terms of three-body nonadditive coefficients. The anisotropy of atom–diatom C ₆ dispersion coefficients arises primarily from nonadditive triple-dipole and quadruple-dipole forces, while pairwise-additive forces and nonadditive triple-dipole and dipole–dipole–quadrupole forces contribute significantly to atom–diatom C ₈ coefficients. The resulting expressions are applied to dispersion coefficients for Li + Li₂ (triplet) and recommendations are made for the best way to obtain global triatomic potentials that dissociate correctly both to three separated atoms and to an atom and a diatomic molecule.

Acknowledgement

PS and JMH are grateful to EPSRC for support under research grant GR/R17522/01. MTC is grateful for sponsorship from the University of Durham and Universities UK.