Some simple methods for calculating van der Waals interactions from oscillator strength sums

A number of approximate procedures for the calculation of long-range two- and three-atom interaction constants are discussed. All the procedures, of which that proposed by Öpik in 1967 is one, result from the inclusion of a first-order static distortion function in a variational trial function of either one- or two-centre type. It is shown that their application requires a knowledge of a limited number of oscillator strength sums (four at the most), and their accuracy is assessed by comparison with previous calculations.