Molecular crystal phonon dispersion curves and model fitting

The procedure for fitting model calculations to measured phonon dispersion curves in molecular crystals is discussed. The simplest model involves the assumption that the molecules behave as rigid bodies. The results for perdeuterohexamethylenetetramine are then fitted to this model by the method of least-squares. The question of molecular distortion in external mode vibration is raised, and the results for a model calculation on perdeutero-naphthalene are given. These suggest that the distortion effect should not be ignored in model fitting procedures, as some modes are found to be shifted by as much as 8 cm–1. The calculations also yield large shifts in the internal mode frequencies and show the dispersion relation for these modes.