Theoretical investigation of intra-atomic electronic excitation energies of divalent Cu in YBa2Cu3O7-x

Ab initio molecular-orbital calculations of small CuO clusters are used to investigate intra-atomic electronic excitation energies of divalent Cu in YBa2Cu3O7-x. In clusters representing the CuO2 planes, all of the Cu d-d excitation energies are found to be in the range 1.3--1.8 eV. The presence of an O 2pπ hole has only a small effect on the excitation energies. Similar results are found for clusters having geometries representing the chains. The relation of the results to experimental optical spectra of YBa2Cu3O7-x is discussed