Nonadditive, three-body dipoles and forces on nuclei: New interrelations and an electrostatic interpretation

Abstract

This work establishes the relation between three-body forces and the polarization P(r), for a group of interacting molecules A...B...C with weak or negligible charge overlap. Using nonlocal polarizability and hyperpolarizability densities, we derive new equations for the three-body forces and dipoles due to dispersion and induction–dispersion interactions. For clusters containing at least one polar molecule, we identify a nonadditive induction–dispersion energy that was not included explicitly in previous analyses of van der Waals cluster spectra. Numerical estimates show that this effect is significant for Ar2...HCl and Ar2...DCl. Separately, we determine the effects of linear and nonlinear induction, dispersion, and induction–dispersion interactions on the polarization P(r); then we relate each contribution to P(r) to the corresponding component of the intermolecular force. Our work provides a classical electrostatic interpretation for all of the three-body forces at long range, including dispersion: The polarization of a molecular cluster produces these forces. This result holds even though particular interaction effects may generate a force but no net dipole, or a dipole but no net force.