Thermal vibrations are accounted for via temperature-dependent scattering phase shifts, as described by Pendry in "Low Energy Electron Diffraction" (Academic Press, London, 1974). (Further details can be obtained in that reference.) These phase shifts represent an atom moving around its equilibrium position. The average square atomic displacement is proportional to the temperature and inversely proportional to the square of the Debye temperature. The latter is a measure of the compressibility of the solid; the higher the Debye temperature the harder the solid. The atomic mass is also involved, but it is stored inside the code for all atomic elements.