We have performed Monte Carlo simulations on the solid phases of nitrogen and improved the nitrogen-nitrogen intermolecular site-site potential derived by Etters el al. Previous simulations with the Etters potential reproduced the properties of dense fluid nitrogen and the structures of the alpha, beta, and delta phase, but failed to describe the low temperature, high pressure gamma and epsilon phases. We have examined the influence of the Coulomb term on the stability of the epsilon phase with various values for the quadrupole and hexadecapole moments, When the quadrupole is taken about 15% larger than the experimental value, the epsilon phase could be stabilized, although the volume and box angle did not correspond to the experimental values. The epsilon 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 epsilon-phase structure, and resulted also in the stabilization of the gamma 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.