Many—Body Aspects of Intermolecular Forces

The many-body aspects of intermolecular forces are examined for the model based on a fluctuating reaction field. This is accomplished by expanding the reaction field in a series involving optical and translational parameters, and an expression is obtained for the cohesive free energy in terms of frequency-dependent polarizabilities and dipole-coupling tensors. When applied to a system of fixed translational parameters a series expansion is obtained for the configurational total potential energy, the leading term of which arises from pair interactions, the next higher term from triple interactions, etc. It is shown that the pair and triple potentials reduce to forms identical with the ones obtained by ordinary quantum-mechanical second- and third-order perturbation theory. The significance of the results is discussed.