Variational formalism and intermolecular forces in OCS lattice dynamics

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Abstract
A variational formalism for lattice dynamics in molecular crystals (consisting of ionic or neutral molecular units) is developed. We take account of the fact that in the presence of large-amplitude motions, the librational and translational degrees of freedom in the crystal are in general not separable, even at the zone center in crystals with centrosymmetric sites. This formalism has been brought to practical realization and applied to the OCS crystal as a test case using an intermolecular potential function for OCS–OCS determined from gas-phase and crystal-phase experimental data. The determination of the potential function yields information on the intermolecular forces in the crystal, and a discussion of these forces is given. Theoretical results for the zero-point energy of the crystal and for the spectroscopic frequency of the ``librational'' transition at zero wave vector are compared with available experimental data. The comparison indicates the essential validity of the intermolecular potential function and the variational formalism employed. It is found that, as expected, a strong coupling exists between the librational and translational degrees of freedom in the crystal. The results obtained show that the methods used are powerful tools for circumventing the harmonic approximation in the lattice dynamics of molecular crystals.