This classic picture is reproduced by unrestricted HF calculations using cluster models, which indicate localization of the hole onto a single defect-adjacent oxygen atom. Hyperfine coupling parameters for 17 O, Si, and Al are consistent with experiment when computed at the HF level, or else using DFT with a large fraction of exact exchange, and the defect exhibits local symmetry breaking and Jahn-Teller reconstruction. Application of semilocal functionals to cluster models, however, affords a hole that is delocalized over all four neighboring oxygen atoms at the Al center, without significant structural distortion. A localized defect is recovered by applying a self-interaction correction, or by using functionals with a large fraction of exact exchange. Notably, B3LYP (with 20% exact ex- change) fails to localize the hole in Al-doped SiO 2 , whereas the functional BB1K (with 42% exact exchange) does localize it, as does DFT+U with a large Hubbard U parameter. Using cluster models shown in Fig. , we have investigated whether DC-DFT is beneficial in describing localization of the hole in Al-doped silica. (Our models are similar to those used in Ref. 25.) Defects are created by replacing a central Si atom with Al and terminal oxygen atoms are capped with hydrogen so that the clusters are charge neutral. All three structures were initially optimized using second-order Møller-Plesset perturbation theory (MP2), with the 6-31G* basis set within the resolution of identity approximation.