The integration of multiplet van der Waals interactions

The integration method for many-particle van der Waals interactions of general order, which was found useful in Drude model calculations can be successfully used also in purely quantum mechanical investigations. The quantum mechanical perturbation series of the dispersion energy is shown not to contain any poles even if the unperturbed electron states are degenerate. It is factorized by means of a linear integral transformation, and subsequently integrated by combining concurrent interactions into effective matrix elements (Green's functions).