Contributions to interatomic and intermolecular forces

Abstract

We have calculated the interaction energy of two He atoms using the VB method, and our most accurate result predicts a van der Waals well depth of 10·05 K. We find that charge transfer and the interaction of interatomic and intra-atomic correlation to make nearly negligible contributions in this system. Our value, ∼0·5 K above the best experimental estimate of well depth, illustrates a feature of our procedure we suggest is universal: calculated potential curves will be an upper bound to the true curve when obtained under suitably restrictive conditions. The problems of superposition or basis set enhancement errors are discussed, and we show how they may be eliminated within the VB framework.