Intermolecular potential models for anisotropic molecules, with applications to N2, CO2, and benzene

Three representations of the angle-dependent intermolecular interactions of nonspherical molecules are compared with one another and with experimental data for nitrogen, carbon dioxide and benzene. The models are the atomic, Kihara core, and overlap. The effect of including quadrupolar terms in the potential functions is also considered. It is found that the theoretical virial coefficients are not greatly altered when the quadrupole terms are added, at least at the temperatures corresponding to the experimental ranges for the molecules chosen. However, the theoretical solid state heat of sublimation and, to a lesser extent, the crystal lattice parameter, do change significantly. Satisfactory fits to the experimental second virial coefficients and the solid state data were obtained by varying ε, σ, the well-depth, and range parameters, while fixing the other constants appearing in the models (such as the parameter specifying the nonsphericity) at reasonable values. Quantities of interest, including the equilibrium distance, angular configuration, and dissociation energy of the van der Waals’ dimer, are estimated from the best-fit potential functions. It is concluded that all three models are approximately equivalent when the molecules are only slightly nonspherical and that there is sufficient flexibility in the models to give good fits to the data even for strongly nonspherical molecules. It appears that the representations of the N2, CO2, and benzene interactions derived here are sufficiently realistic to warrant their use in future work.