Abstract
We have developed a new lattice dynamics sheme for handling large amplitude librational motions or hindered rotations, anharmonic translational vibrations and translation-rotation coupling in molecular crystals. This formalism is based on expanding the intermolecular potential in the molecular displacements and including the cubic and higher terms, while retaining its exact anisotropy. This potential is first used to construct mean field states for the molecular translations and hindered rotations, and next to solve the equations of motion for the crystal in the random phase approximation (RPA), which takes into account the correlations between the molecular motions as well as translation-rotation coupling. It is illustrated that this scheme gives very accurate results for the ordered a and 7 phases of solid nitrogen, while It also yields, for the first time, a quantum dynamical description of the molecular motions in the plastic s phase and a fairly accurate α—s transition temperature.
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