Lattice-dynamical calculation of second-order thermal diffuse scattering in molecular crystals

A computer procedure has been developed to calculate second-order thermal diffuse scattering (TDS) intensity for molecular crystals from lattice-dynamical calculations with an atom-atom potential in the Born-von Kármán formalism. It is applied to monoclinic phenothiazine and different contributions to second-order TDS intensity, acoustic-acoustic, acoustic-optic and optic-optic, are compared. Calculations are also performed in the long-wave approximation allowing for dispersion (LWD) and correction factors of Bragg intensities due to TDS contribution in the LWD approximation are, generally but not always, lower than lattice-dynamical ones; the ratio between LWD and 'exact' factors ranges from 0.4 to 1.4 for reflections considered.