A model for an electron polaron is obtained by adding an extra electron to the Ti 21 O 70 H 56 cluster model, in a manner such that the polaron is associated with the central (bulk-like) Ti atom. Naive approaches to adding this electron cause it to delocalize at the surface of the cluster, which is an edge effect rather than a SIE effect as evidenced by problematic delocalization even at the HF level. Various strategies have been devised to obtain a localized polaron in a finite cluster model, typically involving either bond distortion or else changes to the nuclear charges. Following other theoretical studies of TiO 2 in which Ti-O bond lengths were elongated by 2-8% in order to localize the defect, we opted to elongate all six Ti-O bonds around the central Ti atom in the initial geometry of [Ti 21 O 70 H 56 ] -. The elongated Ti-O bond lengths at the initial geometry can be seen in Fig. and the structure was subsequently relaxed at the HF level, using the Mulliken spin charge s(Ti * ) on the central atom (Ti * ) to monitor (de)localization as the optimization proceeds. This analysis demonstrates that the localized polaron that is initially induced by bond elongation remains localized when the structure is relaxed at the HF level (Fig. ). The density sensitivity at the HFoptimized polaron geometry is S/N atoms ≈ 8 kcal/mol using either BLYP or PBE, which is even larger than was found for the Al-doped silica clusters.