Long-range and overlap effects on collision-induced properties

Abstract

Collision-induced dipole moments and dipole polarizabilities in ion-atom pairs are considered within the framework of the theory of intermolecular interactions. Contributions to collision-induced properties are classified according to their origin in the interaction energy formula. In the long-range electrostatic limit the expansions for dipole-induced-dipole and multipole-induced-multipole models are recovered. The role of different contributions to induced electric properties is investigated numerically for F^- · · ·He and Cl^- · · ·He pairs at the Hartree-Fock self-consistent field and fourth-order many-body perturbation theory correlated levels. Compression of the anion arising from exchange-overlap interaction terms is found to be important in the induced dipole moment, and indispensable for description of the parallel component of dipole polarizability. Overlap effects are, however, strongly anisotropic. As a result, even the simplest electrostatic model is capable of describing the perpendicular component of the induced dipole polarizability over a wide range of separation. Long-range models are expected to fare better for cation-atom pairs.