Phonon dispersion curves and phonon lifetimes in crystalline ammonia

The theory of lattice dynamics of molecular crystals using atom-atom and multipole-multipole potentials developed in previous papers is extended to take account of anharmonic effects. Calculations of the dispersion curves and the density of states for the harmonic phonons in solid ammonia are presented. The finite phonon lifetimes due to anharmonicity are calculated for the zero wave vector modes and compared with experiment. (orig.)