Optical properties of point defects in SiO2 from time-dependent density functional theory

We have studied the optical absorptions of a series of point defects in SiO2 by means of time-dependent density functional response theory (TD-DFT). The structure of the defects has been described with cluster models and atomic orbital basis functions. For each center the lowest singlet --> singlet and singlet --> triplet (for diamagnetic defects) and doublet --> doublet (for paramagnetic defects) transitions have been considered. The results have been compared with accurate ab initio calculations based on explicit treatment of correlation effects and, when possible, with experimental data. Defects with localized wave functions and low excitation energies (<5 eV) are well described by TD-DFT and the predicted transition energies are within a few tenths of an eV from the experimental or ab initio values. For defects which give rise to transition energies higher than 5 eV the TD-DFT values are 10%-20% too low. The comparison of various exchange-correlation functionals show that the hybrid B3LYP method provides more accurate answers than other gradient-corrected DFT approaches. (C) 2002 American Institute of Physics.