Ab initio molecular orbital (MO) calculations have been performed on clusters H6-xSi2O7Na(x) (x = 0, 1, 2, and 4) modeling sodium silicate glasses. Geometries are optimized at the Hartree-Fock/STO-3G level. The 3-21G and 3-21G + polarization d functions on Si basis sets are employed in single-point calculations using the STO-3G optimized geometries. On the basis of the MO diagrams obtained for the clusters, photoionization, X-ray emission, and UV excitation energies are calculated. The calculated energies are in good agreement with the experimental values reported previously. Furthermore, the addition of d orbitals on Si yields a realistic charge distribution in the clusters. The results show that Na atoms in the cluster affect the electron density not only on O atoms next to Na (i.e., nonbridging oxygens) but also on all O and Si atoms in the cluster. Consequently, it is found that Si-O bonds are weakened totally by the introduction of Na into the glass structure. Also, reactivity of the Si-O bonds in sodium silicate glasses are discussed in terms of charge distribution calculated for the clusters.