Van der Waals Potential in Helium

Part I is an investigation of the fundamental question concerning the additivity of first-order exchange and second-order van der Waals potentials. The method employed is a variational one which allows both first- and second-order exchange effects to be calculated simultaneously with the second-order attractive terms. It is shown that the assumption of additivity is entirely false for atomic hydrogen, but that it is almost legitimate for He. By extrapolation one might conclude it to be quite safe for heavier structures. Part II is a redetermination of the numerical values occurring in the attractive part of the potential, based on the use of oscillator strengths ($f$ values) which are adjusted to give the best value for the atomic polarizability of He. It appears that the value of the dipole-dipole coefficient in the Slater-Kirkwood formula has been too high. The final result for the van der Waals potential (Eq. (20)) seems on the whole to be consistent with independent determinations of this quantity from empirical data.