CNDO studies of Y-Ba-Cu-O superconductors I. Comparative study of CuO n polyhedra

Electronic structure of CuO _n ^q clusters (n=2, 4, 5) is calculated in experimental geometries of YBa₂Cu₃O₆ and YBa₂Cu₃O₇ for various formal oxidation states of Cu modelled by total cluster charges,q, both by non-relativistic and by quasi-relativistic CNDO and INDO methods. The results for standard Cu (d¹⁰s¹p⁰) as well as for nonstandard Cu (d⁹s²p⁰) reference configurations are compared from the point of view of the Cu oxidation state. Cu (III) oxidation state seems to be improbable.