A b i n i t i o description of large amplitude motions in solid N2. III. Libron–phonon coupling

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Abstract
A new lattice dynamics scheme is proposed for handling librons, anharmonic translational vibrations, and translational-rotational coupling in molecular crystals. This scheme is an extension of earlier libron models which describe large amplitude librations or hindered rotations. The formalism is based on expanding the intermolecular potential in the molecular displacement coordinates, including cubic and quartic terms, while retaining the exact orientational dependence. Closed expressions are obtained via spherical tensor methods. After constructing separate mean field states for the molecular rotations and translations, using bases of tesseral harmonics and 3D harmonic oscillator functions, respectively, the intermolecular correlations are taken into account and simultaneously the translational-rotational coupling, by solving the equations of motion for the crystal according to the time-dependent Hartree or random-phase approximation. Application of the formalism to the ordered α and γ phases of solid nitrogen, using an ab initio potential, gave very satisfactory results.