SCF Wave functions for (CuO2)n lamella in crystal field of T′–Nd2CuO4

Abstract

AbstractAn SCF analysis has been carried for ensembles that simulate the CuO2 conduction layer in the tetragonal layer crystal T′—Nd2CuO4 (Fig. 1). In this work, the CuO2 layer is described by a planar macromolecule, (CuO2)n, subject to the crystal field produced by the point charges in the ionic layers of D4h symmetry The computations were carried out using the KGNMOL, HONDO, and KGNGRAF codes in MOTECC ‐91. The computations were carried out with different oxygen and copper basis sets and energy convergence to less than 10−8 Hartrees. The purpose of the SCF computation was to estimate the cohesive energy of the ensembles, the electron density for the individual molecular orbitals, and the excess correlation energy, to ascertain the nature of the CuO bond in the conduction layer. The results indicate the following: (1) The cohesive energy of the ensembles (measured by the SCF energy plus the correlation energy, above the atomic values): ΔUc ≡ ΔESCF + ΔEC = −4.35 to −4.17 Hartrees per CuO bond as n increases from 4 to 9. Further insight was obtained by considering the electrostatic energy contributions to ΔUc; Eelectrostatic (ensemble) → EModeling (infinite lattice) were evaluated by replacing the oxygen and copper atoms by point charges determined by a Mullion population analysis. The larger oxygen basis set (13, 8/5,3) gave consistent results for the different ensembles of ΔEcovalent ≡ ΔUc − Eelectrostatic = −1.1 Hartrees per CuO bond. (ii) The electron density indicates that covalent bonds are formed and that the oxygen atoms play an important role in the structure stability. The covalent bonds formed indicate that nominal ionic valences do not apply. Mulliken population analyses gave valences of the order of one at the copper and oxygen atoms. The CuO bond orders are 0.47 between neighboring atoms and 0.048 for those separated by two atoms. (iv) The covalent nature of the CuO bonds in (CuO2)n was compared to that for the H2 molecule using as a measure the electron density and the excess correlation energy. The excess correlation energy per CuO bond above the atomic values is one order of magnitude lower that that for the H2 molecule and that for the CC bond in alternant hydrocarbons. © 1993 John Wiley & Sons, Inc.