AbstractThe work aims at the modelling of molecular spectra of complicated oxide glasses by the ab initio molecular dynamics. As a first step, a classical molecular dynamics simulation of Na2O·3SiO2 silicate glass has been performed to obtain a model of a rather simple glass structure in which some simulation size effects can be evaluated. Molecular vibrations and partial vibrational densities of states have been calculated within the harmonic approximation. Normal modes of vibrations have been projected onto vibrational modes of the symmetrized structure units Si-O-Si, SiO4 and used for the interpretation of experimental Raman spectra. It was found that vibrational spectra do not depend much on a cell size within the studied interval of 114-798 atoms.