Theory of high temperature superconductivity in cuprates: I. First principles cluster calculations for the electronic structures of CuO₆ octahedron and CuO₅ pyramid

In this paper we first describe the multi-configuration self-consistent field method with configuration interaction (MCSCF-CI) which is a variational method most suitable for strongly-correlated cluster systems. Then the calculated results of CuO₆ cluster embedded in hole-doped La_2-xSr_xCuO₄ and of CuC₄ cluster in the electron-doped Nd_2-xCe_xCuC₄ are reviewed. Finally the calculated results on the electronic structure of CuO₅ pyramid embedded in the insulating YBa₂Cu₃O₆ and superconducting YBa₂Cu₃O₇ are newly presented. In the latter case we will clarify an important role of the Cu-O chain in the electronic and superconducting state of YBa₂Cu₃O₇.