Physica C 196 (1992) 340-356 North-Holland PliYSlCA G Ab initio cluster and band structure calculations on systems modeling La2CuO4. Effects of charge transfer between the different planes, Madelung potentials, doping and correlations P. Saalfrank, J. Ladik, R.F. Wood 1, M.A. Abdel-Raouf and C.-M. Liegener Chair for Theoretical Chemistry, Friedrich-Alexander-University Erlangen-Niirnberg, Egerlandstr. 3, D-8520 Erlangen, Germany Received 6 February 1992 Revised manuscript received 28 April 1992 Based on ab initio Hartree-Fock (HF) calculations both on the cluster and band structure level, the electronic structure of La2CuO4 (2-1-4) was investigated. As model systems [ CuO6 ] n- (cluster case) and [CuO+ ] "- 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 Moiler- Plesset many-body perturbation theory. A comparison with density functional (DF) calculations in the local density approxima- tion (LDA) is given. Finally, we studied the effects of doping on the HF level. Here we found that upon doping holes of c 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. 1. Introduction The discovery of high-To superconductivity by Bednorz and Miiller [ 1 ] led to considerable activity in theoretical physics and chemistry in trying to un- derstand this phenomenon. Besides the "usual" for- mation of electron (hole) pairs in the sense of the Bardeen-Cooper-Schrieffer (BCS) [2] theory, completely alternative approaches [3] have been used to explain the facts. Among the BCS-like the- ories pairing based on lattice vibrations [4], elec- tronic charge polarizations (excitons [5 ] ), plas- mons [6 ] ) or spin dependent interactions (spin polarons [ 7 ], spin bags [ 8 ] ) have been proposed. In any case, the key for definitely verifying or rejecting these models is a detailed knowledge of the elec- tronic structure of the high-Tc ceramics. For this purpose both experimental data and the- oretical investigations are required. These are exten- Permanent address: Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. sively reviewed elsewhere [ 9,10 ], so that there is no need to repeat the results in great detail here. At the theoretical side both cluster [ 11 ] and band structure calculations have been performed on various high-To superconductors (HTSCs), most of them making use of the local density approximation (LDA) in con- nection with density functional (DF) hamiltonians [ 12]. Concerning the results of different band struc- ture calculations on La2_xSrxCuO4 (henceforth "2- 1-4") [1,13], YBa2Cu307_y (henceforth "1-2-3" [14,15]), and Bi and T1 containing materials [ 16,17 ], it must be emphasized that there was only partial agreement with early photoemission data [ 18,19 ]. In particular ( 1 ) there was typically a 1-2 eV shift of calculated versus experimental valence-band peaks to lower binding energies [ 20,21 ], (2) satellite structures observed were difficult to ex- plain theoretically [22 ] and (3) photoemission data for instance for YBa2Cu307 showed a notable lack [ 19 ] of the Fermi edge pre- dicted by theory [ 15 ]. 0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.