VANDERWAALS FUNCTIONAL FORMS FOR MOLECULAR SIMULATIONS

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.