Theory of inelastic x-ray scattering by phonons in ice

We calculate the inelastic x-ray scattering phonon line shape in ice. The interaction between molecules is described by the simple point charge potential, averaged over different relative orientations of the molecules. We investigate different mechanisms that can broaden the phonon lines. We find that bond disorder only gives a small contribution to the linewidth. Anharmonicity, on the other hand, gives significant broadening of the phonon peak, and the variation of the width as a function of momentum transfer is in qualitative agreement with experiment. {} 1996 The American Physical Society.