  We investigate the quasiparticle band structure of anatase TiO2, a wide gap
semiconductor widely employed in photovoltaics and photocatalysis. We obtain GW
quasiparticle energies starting from density-functional theory (DFT)
calculations including Hubbard U corrections. Using a simple iterative
procedure we determine the value of the Hubbard parameter yielding a vanishing
quasiparticle correction to the fundamental band gap of anatase TiO2. The band
gap (3.3 eV) calculated using this optimal Hubbard parameter is smaller than
the value obtained by applying many-body perturbation theory to standard DFT
eigenstates and eigenvalues (3.7 eV). We extend our analysis to the rutile
polymorph of TiO2 and reach similar conclusions. Our work highlights the role
of the starting non-interacting Hamiltonian in the calculation of GW
quasiparticle energies in TiO2, and suggests an optimal Hubbard parameter for
future calculations.
