Ab initio cluster and band structure calculations on systems modeling La₂CuO₄. Effects of charge transfer between the different planes, Madelung potentials, doping and correlations

Based on an initio Hartree-Fock (HF) calculations both on the cluster and aand structure level, the electronic structure of La₂CuO₄ (2-1-4) was investigated. As model systems [CuO₆]^n- (cluster case) and [CuO₄]^m- units (band structure case) were chosen, i.e. the "apical" oxygens were explicitly included. Surrounding ions were treated as point charges (cluster case) and via partially self-consistent Madelung potentials (band structure case), respectively. Using the HF approximation we found a too low density of states (DOS) around the Fermi level, while the charge distribution in the ground state seems to be reasonable. To obtain DOS curves comparable with the experimental ones, we had to go beyond the one-particle description by using Møller-Plesset many-body perturbation theory. A comparison with density functional (DF) calculations in the local density approximation (LDA) is given. Finally, we studied the effects of doping on the HF level. Here we found that upon doping holes of ω symmetry and predominant oxygen character are formed within the CuO₂ planes of 2-1-4. At the same time increasing covalent bonding between in-plane Cu and O was observed.