WHMO calculations on CuO clusters of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB>: correlation between T<SUB>c</SUB>, structure, oxygen stoichiometry and highest occupied molecular orbital energy

WHMO calculations of reasonably big CuO clusters, namely, Cu<SUB>12</SUB>O<SUP>&#8722;42</SUP><SUB>33</SUB> and Cu<SUB>9</SUB>O<SUP>&#8722;46</SUP><SUB>32</SUB> are used to investigate the correlation between highest occupied molecular orbital (HOMO) energy and T<SUB>c</SUB>. We observe excellent correlations between HOMO energy and oxygen stoichiometry and in turn T<SUB>c</SUB> and structure. It is demonstrated that CuO<SUB>2</SUB> planes are essential for superconductivity for the YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> system. Fe<SUP>3+</SUP>, Zn<SUP>2+</SUP> and Ni<SUP>2+</SUP> doped CuO clusters are also investigated, and a good correlation between HOMO energy and oxygen stoichiometry is observed. Site preference of the dopants is also analyzed