Note on the Calculation of van der Waals Forces

Abstract

The general expression for the van der Waals energy arising from dipole interaction between atoms involves a summation over transition probabilities. These are in most cases difficult to calculate. It is here pointed out that since these transition probabilities obey the sum rule of Thomas and Kuhn, they may be evaluated in a manner which permits van der Waals' forces to be calculated with good approximation, provided that the polarizability is known. The method is outlined and illustrated by application to two examples: Na and He. The result in the case of He is in very good agreement with that obtained by the variational method.