Quasiparticle band structure of SrTiO3 and BaTiO3: A combined LDA+U and G0W0 approach

We present the quasiparticle band structures of prototypical oxide perovskites SrTiO3 and BaTiO3, two seemingly simple oxides for which accurate calculations of the electronic structure have been met with significant (and somewhat unexpected) challenges. Previous G0W0 calculations predicted a band gap ranging from 3.36 to 3.82 eV for SrTiO3, with a majority of the studies giving a band gap around 3.7 ∼ 3.8 eV, to be compared with the experimental value of 3.25 eV. A similar discrepancy between theory and experiment is also observed for BaTiO3. We show that the G0W0 approach can predict reasonably accurate band gaps of SrTiO3 and BaTiO3, provided that the calculations are carried out on top of the local density approximation (LDA) plus U (LDA+U) solutions and are fully converged. The deficiency of the LDA in describing the localized 3d states, although not particularly critical in this case, still results in a poor mean-field starting point for subsequent many-body perturbation calculations. G0W0 calculations starting from the LDA+U solutions, on the other hand, give significantly improved results for both systems. Our work demonstrates the accuracy and applicability of the combined G0W0 and LDA+U approach in calculating the quasiparticle band structures for materials involving localized 3d states, not only for systems with fully occupied 3d semicore states as has been shown before, but also for systems in which the 3d states are nominally unoccupied.