Molecular dynamics in crystallineα-nitrogen

Harmonic lattice-dynamics and classical molecular-dynamics (MD) calculations for crystalline {α}-nitrogen were used to study a semiempirical potential (5q) for the interaction of pairs of nitrogen molecules. The dispersion curves along the special directions of {α}-nitrogen, and the density of states for the crystal have been calculated in the harmonic approximation, and comparisons made with appropriate experimental data. The effects of increasing temperature at constant volume were studied using MD, mainly through the calculated Raman spectrum. Quite modest shifts relative to the harmonic frequencies were obtained. Further studies with a model potential used by Kobashi in anharmonic perturbation calculations showed similarly modest shifts, contrary to the predictions of perturbation theory. An analysis of the molecular motions showed that for both models the librational amplitudes are large, and we conclude that the perturbation-theory predictions probably overestimate the shifts substantially. Complete reorientations of molecules (flips) were observed in the highest-temperature run for the 5q model, but a detailed study failed to reveal a specific mechanism of reorientation.