Intermolecular Forces: What can be Learned from Ab Initio Calculations?

Abstract

Various experiments, such as elastic or rotationally inelastic molecular beam scattering(1, 2) and spectroscopic studies of so-called Van der Waals molecules(3, 4), have been designed especially to provide information about the Van der Waals interactions between molecules. The results of these measurements, as well as other experimental data obtained on bulk materials, e.g. the phonon frequencies in molecular crystals(5), depend very sensitively on the shape of the intermolecular potentials. Still, it is not easy to extract the potentials from these data. One has to assume parametrized model potentials of a certain analytic form and to fit the parameters such that calculations yield the best agreement with the measured quantities. Often, this does not lead to unique and accurate values for all the parameters and, moreover, the model potentials assumed may have a form which is not completely correct and not sufficiently flexible. Therefore, it is very useful that, for smaller systems, information about the intermolecular potentials can also be obtained from ab initio quantumchemical calculations.