All-electron GW calculation of rutile TiO2 with and without Nb impurities.

The all-electron mixed basis GW approach is applied to calculate the electronic structure of rutile TiO2 and Ti0.75Nb0.25O2. The quasiparticle energies are calculated in fine detail, allowing the accurate determination of electronic structures. Although the valence band maximum (VBM) of the pure rutile TiO2 is composed of O 2p and Ti 3d orbitals, that of Ti0.75Nb0.25O2 is composed of Nb 4d orbitals and O 2p orbitals together with a few Ti 3d orbitals, forming an occupied impurity level, while the conduction band minimum is composed of Nb 4d and O 2p orbitals. In the calculation of Ti0.75Nb0.25O2, the energy level of the VBM increases, with finite energy gap, in contrast to the local density approximation calculation showing a metallic band structure. The exchange and correlation contributions to the self-energy drastically change in the VBM. The resulting electronic structure is in reasonable agreement with experiments. Based on our results, the GW approximation seems good in describing a not so strongly correlated transition metal oxide system with impurities. [ABSTRACT FROM AUTHOR]