We have carried out ab initio molecular orbital calculations on a cluster of atoms containing five SiO3/2O-Na+ units connected at the corners by siloxane bonds to investigate the structure and vibrational properties of sodium disilicate glass. The optimized structural parameters (mean Si-O, O-O, and Na-O bond distances and O-Si-O bond angles) and O 1s photoelectron spectrum calculated at the Hartree-Fock/6-31G(d) level are in excellent agreement with the observed ones. We have also performed frequency calculations using the optimized cluster and have calculated their infrared and Raman intensities. The calculated vibrational spectra agree well with those observed for sodium disilicate glass. On the basis of the calculated results, we interpret the vibrational spectra of sodium disilicate glass and discuss the local coordination environments of sodium ions in the glass.