Far-Infrared Laser Spectroscopy of van der Waals Bonds: A Powerful New Probe of Intermolecular Forces

“Molecular mechanics”-large-scale computer modeling of molecular structure and function-has become one of the most rapidly expanding activities in chemistry and biology. As increasing numbers of scientists turn to these semiempirical computer codes in order to design new drugs, catalysts, artificial enzymes, etc., our incomplete knowledge of the. “weak interactions” of chemistry-van der Waals forces and hydrogen bonding (collectively termed intermolecular forces in this article)-is becoming a progressively more serious impediment. Intermolecular potentials are a critical part of the “input” required for molecular mechanics codes, as well as for statistical mechanics and molecular dynamics models of chemical reactions in liquids, gas-surface interactions, and protein dynamics. Even the best such calculations are based on simplistic “6-12 type” forms for the intermolecular potential surfaces (i.e., with isotropic well depths and an average van der Waals distance), usually incorporating the additional assumption of pairwise additivity of intermolecular forces, i.e., neglecting three-body and higher terms in the potential.