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Multiple small samples of amorphous silica have been generated and optimized using classical dynamics and the van Beest-Kramer-van Santen ͑BKS͒ empirical potential function. The samples were subsequently optimized and annealed using density functional theory ͑DFT͒ with both the local density and the generalized gradient approximations. A thorough analysis of the local and medium-range structure of the optimized samples obtained from the different methods was carried out. The structural characteristics obtained for the average of small systems each containing ca. 100 ions are compared for each of the different methods, and to the BKS simulation of a larger system. The differences found between the DFT and BKS simulations and the effects of volume relaxation on the structures are discussed. Fixed-volume samples are compared to neutron scattering data, with good agreement to 5 Å, the length limit of the sample sizes used here. It is shown that by creating multiple small samples, it is possible to achieve a good statistical sampling of structural features consistent with larger simulated glass systems. This study also shows that multiple small samples are necessary to capture the structural distribution of silica glass, and therefore to study more complex processes in glass, such as reactions.

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The structure and properties of self-trapped excitons ͑STE͒, were investigated using density functional theory ͑DFT͒ and wave function-based ͓UHF, UMP2, CAS-SCF, and CCSD͑T͔͒ electronic structure methods. The DFT results were compared to those obtained using the different wave function-based electronic structure methods that treat the electron correlation and exchange with varying degrees of accuracy. The calculations were carried out on cluster configurations extracted from supercell DFT calculations of the STE in ␣-quartz. Two luminescent STEs were found, as well as a nonradiative state at a crossing of the singlet and triplet surfaces. One of the luminescent STEs is the same as that previously found by Fisher, Hayes, and Stoneham ͓J. Phys.: Condens. Matter 2, 6707 ͑1990͔͒. It was furthermore determined that the PW91 functional underestimates the energy of the triplet state, and that this error is greater with greater delocalization of the excess spin density of the state.

We compute the distribution of electronic levels of native defects in amorphous silica from total energy differences of charge-state density functional theory calculations over an ensemble of atomic structures. The predicted distributions reproduce results from trap spectroscopy by charge injection experiments, validating the calculations. Furthermore, our study characterizes the experimentally inaccessible contributions of individual defect types to the overall distribution. Computed electron and hole trapping levels provide insight into the positive charge buildup in bulk silica observed in negative-bias-temperature-instability, an important degradation mechanism of metal-oxide-semiconductor devices.