Long-Range Intermolecular Forces

Abstract

The calculation of long-range intermolecular forces is systematized so as to apply to molecules in electronically excited states where resonance as well as electrostatic, induction and dispersion forces are encountered. Especial attention is given to deviations from the pairwise additivity of the energy of interaction of systems involving three or more molecules. Three types of nonadditivity are found. These are due to resonance degeneracy, molecular degeneracy, and multipole-induction effects. The following specific examples are considered: (a) the interaction energy of three hydrogen atoms, two in the ground state and the third in the first excited state; (b) the interaction of two polar, nondegenerate molecules with a hydrogen atom in its first excited state; and the multipole induction nonadditive interaction between (c) three polar molecules, and (d) three quadrupolar molecules. Throughout these calculations the third and higher order terms in the perturbation are neglected.