Computational Consequences of Neglected First-Order van der Waals Forces

Abstract

We have computed the widely neglected first-order interaction between neutral atoms. At interatomic separations typical of condensed media, it is nearly equal to the 1/r^6 second-order London energy inferred from interactions in gasses. Our results, without the exchange forces that lead to covalent bonding, suggest that the quality of non-bonding attraction between neutral atoms of molecules in condensed media differs from the 1/r^6 form usually ascribed to it. If we add first-order and all second-order terms, we obtain energies nearly five times the 1/r^6 London energies which dominate only at the atomic separations found in gasses. For computation, we propose a practical, accurate form of energy to replace the qualitatively inaccurate Lennard-Jones and harmonic forms casually assumed to hold at the interatomic separations found in condensed media.