Dynamic structure factor of vitreous silica from first principles: Comparison to neutron-inelastic-scattering experiments

Using a first-principles approach, we study the vibrational properties of vitreous (Formula presented) which are measured in neutron-scattering experiments. We adopt a model structure consisting of corner-sharing tetrahedra, which was previously generated using first-principles molecular dynamics. We calculate the dynamic structure function (Formula presented) as a function of wave vector (Formula presented) and energy (Formula presented) by taking explicitly into account the correlations between different atoms as given by the normal modes. The effects of temperature and finite displacements are also considered. Overall, the agreement with experiment is very good, as illustrated by the comparison for the density of states. However, the calculated and measured (Formula presented) differ in some cases up to a factor of 2 in absolute intensity. Nevertheless, the oscillations in (Formula presented) describing the correlations between the motions of the atoms are accurately reproduced. The neutron effective density of states obtained directly from (Formula presented) yields a good representation of the actual density of states. By introducing a comprehensive scheme, we clarify the relation between neutron and infrared spectra. In particular, we show that the neutron density of states does not distinguish between longitudinal and transverse excitations. Other properties such as the mean-square displacements and the elastic structure factor are also evaluated and found to be in good agreement with experiment.