The use of a molecular model to calculate mechanically and electrically induced phonon shapes in molecular crystals is discussed. The model rests on the defect space assumption also made in the standard use of Greenrsquo;s functions. It has, however, the merits to be much more simple and transparent in terms of the site group symmetry. Comparison with complete fcc latice calculations (Ar, N2, CO) shows a rapid convergence of weighted and unweighted densities of states with number of shells. Raman and allowed and forbidden IR spectra are presented as further illustrations. One exception to rapid convergence is found for mechanically coupled phonon bands involving very specific breathing motions.
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