Carrier location in high-Tcsuperconductors 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 Cu${\mathrm{O}}_2$ sheets. Because the virtual state involving the copper $d^{10}$ 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 $p\sigma$ orbital, rather than by the $p\pi$ orbital which is favored by the Madelung potential. Our previous conclusions concerning the carrier quasiparticles are supported by these calculations.