The optical properties of a material critically depend on its defects and understanding them requires substantial and accurate input from theory. In our group we aim to model the optical properties of color centers with ab initio methods.
The standard local approximations of density functional theory with wide band gap materials such as the alkali halides fail and other numerically more expensive methods have to be used. Therefore it is of crucial importance to find a compact basis set that describes the defect states well. We have put considerable effort into finding this basis set employing techniques of filtering out the virtual orbitals of the defect crystal that are overlapping with the orbitals of the crystal without defect.