Self-Energy and Excitonic Effects in the Electronic and Optical Properties of TiO2 crystalline phases

We present a unified ab initio study of electronic and optical properties of TiO2 rutile and anatase phaseswith a combination of density-functional theory and many-body perturbation-theory techniques. The consistenttreatment of exchange and correlation, with the inclusion of many-body one-particle and two-particles effectsin self-energy and electron-hole interaction, produces a high-quality description of electronic and opticalproperties, giving, for some quantities, the first available estimation for this compound. In particular, we givea quantitative estimate of the electronic and direct optical gaps, clarifying their role with respect to previousmeasurements obtained by various experimental techniques. We obtain a description for both electronic gapand optical spectra that is consistent with experiments by analyzing the role of different contributions to theexperimental optical gap and relating them to the level of theory used in our calculations. We also show thespatial properties of excitons in the two crystalline phases, highlighting the localization character of differentoptical transitions. This paper aims at understanding and firmly establishing electro-optical bulk properties, yetto be clarified, of this material of fundamental and technological interest for green energy applications.