  New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient $C(\omega)$ and, in particular, its low frequency
limit. This study indicates that in the $\omega \to 0$ limit $C(\omega)$
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.
