Density functional theory study of impurity levels in Cr-substituted alumina at different Hubbard parameters

A theoretical model of the electronic structure of chromium-substituted aluminum oxide (CrₓAl₂₋ₓO₃) is needed to understand the optoelectronic properties of ruby from first principles. However, simple density functional theory (DFT) is unable to completely describe the localized d-electron states of transition metal impurities because of its limited treatment of strong electron correlations. To address this problem, one can employ a Hubbard U correction (DFT+U) to account for the energy cost of double occupation.