  We present a unified ab-initio study of electronic and optical properties of
TiO2 rutile and anatase phases, with a combination of Density Functional Theory
and Many Body Perturbation Theory techniques. The consistent treatment of
exchange-correlation, with the inclusion of many body one-particle and
two-particles effects in self-energy and electron-hole interaction, produces a
high quality description of electronic and optical properties, giving, for some
quantities, the first available estimation for this compound. In particular, we
give a quantitative, direct evaluation of the electronic and direct optical
gaps, clarifying their role with respect to previous values obtained by various
experimental techniques. We obtain a description for both electronic gap and
optical spectra that is consistent with experiments, analysing the role of
different contributions to the experimental optical gap and relating them to
the level of theory used in our calculations. We also show the spatial nature
of excitons in the two crystalline phases, highlighting the localization
character of different optical transitions. This paper aims at understanding
and firmly establishing electro-optical bulk properties, so far not yet
clarified, of this material of fundamental and technological interest for green
energy applications.
