Influence of wave-function updates in GW calculations on titanates

We investigate quasiparticle band structures of rutile and anatase phases of TiO2 and cubic SrTiO3 using various levels of GW approximations such as G(0)W(0), GW(0), GW, QPGW(0), and QPGW. It is found that the quasiparticle band gap increases as the self-consistency level is ratcheted up from G(0)W(0) to GW(0) and GW, and the GW results overestimate the band gap by similar to 50% in comparison with the experimental values. Interestingly, when the wave functions are updated in QPGW(0) and QPGW, the band gap is significantly reduced, which is opposite to the tendency known for other materials. The experimental band gap is most closely reproduced by QPGW(0) within similar to 10% error. We explain the improvement by the wave-function update in terms of the local change in the charge density, which is different at Ti and O sites and reduces the quasiparticle energy gap.