Electronic structure of undoped and dopedLa2CuO4: A Hartree-Fock cluster study

By use of the ab initio Hartree-Fock method, ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ in undoped and doped form is studied by cluster calculations that take the La atoms explicitly into account. In these computations the important effects of the crystal field are studied in detail. Density-of-states curves are computed and compared with experiment and other calculations. The charge and spin distributions and the extent of covalent bonding in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ are examined: We have found the bonding to be nearly ionic. Covalency becomes more important if one La atom in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ is replaced by one of the alkaline-earth elements (Ba, Sr, Ca, or Mg). This ‘‘doping’’ produces holes in the ${\mathrm{CuO}}_2$ planes that are mainly oxygenlike and are of σ symmetry. The alkaline-earth atoms are fully ionized, thus suggesting that theoretical models based on the polarizability of these substituents may not be valid.