Ab initio MO Studies of the Hole Delocalization in Copper Oxides and Related Species: Necessity of the Extended Hubbard Model

Abstract
The positive holes for copper oxides and isoelectronic species were investigated by the ab initio molecular orbital (MO) method. It is found that the hole is delocalized over the oxide clusters because of the strong 3d σ-2p σ hybridization, and that the spin polarization of the closed-shell pairs is remarkable, indicating an important role of the spin fluctuation for high-T c superconductivity. The extended Hubbard model is necessary for theoretical studies of the hole delocalizations in copper oxides.