Anharmonic effects in the phonon spectra of sodium chloride

Results are reported of a series of molecular-dynamics "experiments" on solid NaCl at temperatures of 80, 302, 954, and 1153 K. Attention is focused primarily on the computation of the dynamical structure factor and its one-phonon approximation; comparison of the two allows the isolation of contributions from multiphonon and interference terms. Anharmonic effects are analyzed in terms of perturbation theory and the theory is found to give satisfactory results for the phonon frequencies at temperatures up to 80% that of melting. Most of the calculations are carried out for a simple rigid-ion potential, but the effects of polarization and its incorporation in molecular-dynamics calculations are briefly discussed.