Statistical Theory of Low Frequency Intermolecular Forces

The sum of states of a gas consisting of optically anisotropic polar molecules is shown to be substantially equal to the classical Gibbs phase integral at ordinary temperatures. A general method of obtaining the quantum correction to be applied at low temperatures is developed. The intermolecular potential energy appearing in the phase integral is semi-classical in form. The low frequency contribution arising from the permanent dipole moments of the molecules is exactly equal to their mutual electrostatic energy. The high frequency contribution is given by a modification of London's formula in which continuous functions of the molecular orientations occur instead of spatial quantum numbers.