Cluster models for the photoabsorption of divalent defects in silicate glasses: Basis set and cluster size dependence

The two lowest singlet excited states of divalent Si defects in silicate glasses are studied by ab initio methods. Excitation energies of 5.2 and 6.8 eV are obtained. Steady convergence to these values is shown with the increase in size both of the model cluster and of the basis set employed in the calculations. The results clearly demonstrate the viability of the quantum-chemical cluster approach for the study of local excitations in glass defects.