Hole trapping at Al impurities in silica: A challenge for density functional theories

The atomic geometry and electronic structure around a neutral substitutiona l Al impurity in silica is investigated using either the unrestricted Hartr ee-Fock (UHF) approximation, or Beckes three-parameter hybrid functional (B 3LYP). It is found that the B3LYP functional fails to describe the structur al distortions around the Al impurity, while the UHF results are consistent with experimental information. We argue that the failure of the B3LYP func tional is caused by the incomplete self-interaction cancellation usually pr esent in density functional theories.