  We have determined the vibrational properties of a sodium tetrasilicate
(Na$_{2}$Si$_{4}$O$_{9}$) glass model generated by molecular dynamics
simulations. The study has been carried out using a classical valence force
fields approach as well as an {\it ab initio} approach in the framework of the
density functional theory. The total and partial vibrational densities of
states (VDOS) are presented, as well as some characteristics of the vibrational
modes (participation ratios, correlation lengths). For the low-frequency bands
below 500 cm${}^{-1}$, we find that the shapes of the two calculated VDOS as
well as those of their corresponding partial VDOS are quite similar. For the
intermediate- and high-frequency ranges, we observe larger discrepancies
between the two calculations. Using the eigenmodes of the dynamical matrix we
also calculate the polarized Raman spectra within the bond-polarizability
approximation. We find an overall agreement between the calculated parallel
polarized (VV) Raman spectra and the corresponding experimental spectrum.
Regarding the perpendicular depolarized (VH) Raman spectrum, the comparison of
the calculated spectra to the experimental data indicates a need for an
adjustment of the VH bond-polarizability parameters.
