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 SiO2 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 dynami cs. We calculate the dynamic, structure function S(q,E) as a function of wave vector q and energy E by taking explicitly into account the co rrelations between different atoms as given by the normal modes. The e ffects of temperature and finite displacements are also considered. Ov erall, the agreement with experiment is very good, as illustrated by t he comparison for the density of states. However, the calculated and m easured S(q,E) differ in some cases up to a factor of 2 in absolute in tensity. Nevertheless, the oscillations in S(q,E) describing the corre lations between the motions of the atoms are accurately reproduced. Th e neutron effective density of states obtained directly from S(q,E) yi elds a good representation of the actual density of states. By introdu cing a comprehensive scheme, we clarify the relation between neutron a nd infrared spectre. In particular, we show that the neutron density o f states does not distinguish between longitudinal and transverse exci tations. Other properties such as the mean-square displacements and th e elastic structure factor are also evaluated and found to be in good agreement with experiment.