Carrier location in high-Tc superconductors and the degree of cu-o covalency

We exactly solve a realistic three-band extended Hubbard model for the ground and low-lying excited states of a variety of finite clusters representing CuO2 sheets. Because the virtual state involving the copper d10 valence fluctuation has its energy sharply lowered by the presence of a carrier hole, a strong stabilization results if the carrier holes are in oxygen orbitals that are strongly coupled to the copper sites: For example, in spite of strong correlation, at carrier densities of interest and with a reasonable choice of energy parameters, the carrier hole wave function is dominated by the porbital, rather than by the p orbital which is favored by the Madelung potential. Our previous conclusions concerning the carrier quasiparticles are supported by these calculations.