Calculation of van der Waals forces

CHED 949

Anna N. Bloom, and Paul D. Schettler, Department of Chemistry, Juniata College, 1700 Moore Street, Huntingdon, PA 16652
We have developed a quantum Monte Carlo (QMC) which calculates van der Waals forces between neutral gas-phase molecules. While previous QMC work concentrated on total molecular energies, our approach focuses on only those terms associated with van der Waals forces. We examined helium-helium, argon-argon, and ethylene-ethylene interactions by a variety of HF methods dealing with electron correlation up to the MP4 level. These results, in the form of energy vs. distance of separation curves, were compared with experiment. Our QMC method uses one fitted parameter which is adjusted to obtain nearly exact agreement with experiment, at least for helium and argon. With the caveat of the adjustable parameter, our QMC method was both faster and more accurate than the standard Gaussian methods.