As a second application, we studied polaron localization of an extra electron in a sizable cluster model of bulk anatase TiO 2 . Starting from a HF-optimized structure with a localized polaron, we find that optimization with PBE0 or HSE06 preserves the defect localization whereas optimization with PBE drives delocalization onto multiple Ti atoms, leading to elongation of multiple Ti-O bonds. As was the case in Al-doped silica, this GGAinduced structural distortion is severe enough that a subsequent PBE0 single-point calculation cannot recover a localized polaron, because exaggerated bond dipoles over multiple stretched bonds cause a fractionation of the spin density. This is a significant observation, insofar as a standard procedure in computational materials science is to perform structure relaxations using a GGA (often PBE) followed by single-point energy calculations with a hybrid functional (often PBE0 or HSE06). In each of the examples considered here, that procedure affords qualitatively incorrect electronic structure.