Effect of improved band-gap description in density functional theory on defect energy levels in alpha-quartz

We compare defect energy levels of point defects in alpha-quartz using density functionals of different accuracy: a standard generalized-gradient-approximation functional and a hybrid functional. The latter is known to improve the description of band gaps through the incorporation of a fraction of Hartree-Fock exchange. The separations between the defect levels are found to be preserved when going from the standard to the more accurate hybrid functional, indicating that they are already well described with the standard functional. We also find that the defect wave functions obtained with the two functionals are very similar. Consequently, the defect energy levels pertaining to the hybrid functional can be obtained through the perturbational method. (C) 2007 Elsevier B.V. All rights reserved.