Structural and Optical Properties of Point Defects in α-SiO₂ Cluster^*

First-principles methods based on the density functional theory (DFT) are used in order to calculate the structural and optical properties of α-SiO₂ cluster with the non-bridging oxygen hole centers (NBOHC) and NBOHC-E' defects. We clarify the stable structure of the NBOHC-E' point defects for the first time using the functional B3LYP, which is also tested to investigate the influence of electronic properties. The calculation is carried out for cluster configurations extracted from supercell. The results of optical absorption peak for Si₂O₇H₆ and Si₅O₁₆H₁₂ with NBOHC-E' defects are found at 2.66 eV, which is higher than the often observed OA peak at 2.0 eV for the NBOHC defect in α-SiO₂. The overall absorption spectra are in qualitative agreement with the experiment.