Deviations from Additivity of the Intermolecular Field at High Densities

The intermolecular field of an assembly of neutral atoms or molecules is usually assumed to be additive with respect to isolated pairs. Nonadditive effects have been calculated in the first-order forces by Rosen and in the second-order forces by Axilrod and Teller. For the evaluation of second-order forces between two molecules at high densities the zero-order wave function must be made antisymmetric with respect to nearest neighbors. It is shown that this correction results in a decrease of the second-order forces compared to an isolated pair of molecules, and may be interpreted as a screening effect. The method is based on the model of the caged atom or molecule.