Molecular orbital calculations on a cluster model representing the unit cell of YBa2Cu3O7−δ showed that the fully oxygenated sample exhibited large HOMO–LUMO energy gaps but with the oxygen O(1) vacancy the appropriated superconducting gaps were obtained.

Molecular Orbital Analysis of Oxygen Vacancy in YBa2Cu3O7-δ. J.A.M.R.Rocha, A.C.Pavão: Physica C, 2004, 411[3-4], 148-51