Defect states arise from unpaired electrons which are created by point defects. Within the framework of DFT+U, the defect state energy location can be directly correlated to the choice of the U parameter. As noted in the main text, low U values produce defect states at the bottom of the conduction band, while larger U values push the defect energies into the gap. A comparison of several defects and their location relative to the conduction band is shown in Figure S.1. The plot indicates that the defect state location varies nearly linearly as a function of U, regardless of the type of defect (i.e. HO b or O V ).

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Hydroxyls on a TiO 2 surface and photoinduced e -polarons give rise to excess charges, the electronic structure of which is critical to the fundamental understanding of their role in the reactivity of surface absorbates and other photochemical processes. In this paper, we report on a DFT+U characterization of the electronic structure of one excess electron in bare and singly hydroxylated rutile (110) surfaces. The excess electron has the electronic structure of a small polaron with its spin density and associated lattice distortion localized around a single site. Calculations indicate that the most stable Ti trapping site in both bare and hydroxylated surfaces resides in the first subsurface layer under the Ti 5c row. However, trapping energy differences between several Ti sites are within 0.2 eV, indicating that the Boltzmann population of these sites is significant at room temperature and that the excess electron will appear as fractionally occupying several sites. On the basis of earlier calculations, the activation barrier for electron hopping from site to site is small (<0.1 eV). The stability ordering of the different Ti sites is very similar for the bare and hydroxylated surface, suggesting that the hydroxyl only weakly perturbs the surface electronic structure.