van der Waals functional forms for molecular simulations

Abstract

We derive the general functional form for the van der Waals interaction of two 1s orbitals without resorting to approximating the interaction Hamiltonian by a power series expansion. We find the functional form to be the difference between two Extended Rydberg functions. The Morse function offers a good approximation for it. Reference potentials for several small systems are fit with the Morse, exponential-6, and Lennard-Jones functional forms. The Morse functional form describes nonbonded interactions quite well, while the more conventional functional forms possess the wrong shape.