Convergence of the multipole results for first order orientation dependent intermolecular forces using NH3-H+as a model interaction

Abstract

The NH₃-H⁺ interaction is used as a model for discussing the usefulness of energy multipole expansions for interactions where the first order energy plays an important role. The multipole results for the first order energy are analysed formally and are compared numerically with the non-expanded first order energy for a variety of NH₃-H⁺ relative configurations. The results are used to discuss the limitations of the multipole expansion of the first order energy which can be very severe for some NH₃-H⁺ collision trajectories.