Résumé
We compare the vibrational properties of model SiO_2 glasses generated by
molecular-dynamics simulations using the effective force field of van Beest et
al. (BKS) with those obtained when the BKS structure is relaxed using an ab
initio calculation in the framework of the density functional theory. We find
that this relaxation significantly improves the agreement of the density of
states with the experimental result. For frequencies between 14 and 26 THz the
nature of the vibrational modes as determined from the BKS model is very
different from the one from the ab initio calculation, showing that the
interpretation of the vibrational spectra in terms of calculations using
effective potentials can be very misleading.