The importance of the anisotropic energy term for the structure of the solid phases of nitrogen

We have performed Monte Carlo simulations on the solid phases of nitrogen and improved the nitrogen–nitrogen intermolecular site–site potential derived by Etters et al. Previous simulations with the Etters potential reproduced the properties of dense fluid nitrogen and the structures of the α, β, and δ phase, but failed to describe the low temperature, high pressure γ and ε phases. We have examined the influence of the Coulomb term on the stability of the ε phase with various values for the quadrupole and hexadecapole moments. When the quadrupole is taken about 15% larger than the experimental value, the ε phase could be stabilized, although the volume and box angle did not correspond to the experimental values. The ε phase could also be stabilized by introducing an anisotropic term, which influences the shape of the molecule. The two anisotropy parameters have been determined with respect to the ε-phase structure, and resulted also in the stabilization of the γ phase. The anisotropic term changes the shape of the nitrogen molecule by changing the major diameter of the prolate spheroid, which is in agreement with electron density calculations.