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

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 + Li-2 (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.