Lattice dynamics of solid nitrogen with an ab initio intermolecular potential. II. Anharmonic librations in the α phase

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Abstract
Using an intermolecular N2–N2 potential from ab initio calculations, we have performed calculations of frequencies for anharmonic librational modes at zero wave vector in the α phase of solid nitrogen. In the first step, the librations (Eg, Tg, Tg) were considered as uncoupled oscillators with frequencies calculated from an effective one-dimensional anharmonic potential being a section of the crystal potential surface along a direction specified by harmonic eigenvectors of a given mode. Within the approximation, very good agreement with the experimental frequency for the Eg mode has been found. Finally, the coupling between two Tg librational modes has been introduced and the calculated frequencies were found to be almost identical to the experimental ones.