The local coordination environments of alkali cations (Li+ and Na+) in single- and mixed-alkali silicate glasses have been investigated by using cluster model calculations at the Hartree-Fock/6-31G(d) level. The optimized structural parameters obtained for the model clusters are in good agreement with the observed ones. It has been demonstrated that the average coordination number of alkali cations increases in going from Li to Na and that each alkali cation is located in rather individual local sites in both the single- and mixed-alkali clusters. We have further calculated the site-mismatch energies and have shown that a considerable site mismatch is possible to occur during hopping events of alkali cations in mixed-alkali glasses. The present calculated results have elucidated that the molecular orbital overlap or the covalent interaction between alkali cations and their surrounding oxygen atoms plays an important role in structuring individual cations sites, indicating that the physical origin of the mismatch energy in mixed-alkali glasses should not be attributed simply to ion-size differences.