Towards an intermolecular potential for nitrogen

Abstract

An intermolecular pair potential energy function has been developed for nitrogen which is able to reconcile theoretical results for the system with experimental data for various bulk properties. The long and short range forms of the potential are known from quantum mechanical calculations. Modifications to the quadrupole-quadrupole energy due to electron charge overlap effects have been included. In the well region, many different representations have been studied and have been assessed by fitting to experimental data, chiefly for the low temperature lattice properties and second virial coefficients. A number of functions have been derived which are able to reproduce the available information within experimental error. Corrections for non-additive three body terms have been included in calculating the lattice properties, and the first order quantum corrections to the virial coefficients were also included. The dilute gas shear viscosity (based on the Monchick-Mason approximation) was also calculated and used to discriminate between different possible potential energy functions. Other properties such as the isothermal comressibility of α-N₂ at 0 K and the α-γ phase transition at high pressure were also studied and provided further tests of the best potential. The minimum energy dimer configuration was also investigated. We have also tried to represent our model potential analytically in the form of an expansion using spherical harmonics.

It is believed that the potential function derived here is superior to other proposed models and that the techniques described should form a useful basis for investigations of the intermolecular pair potentials of other simple molecular systems.