Electron Correlations in the High Tc-Compounds

Ab-initio correlation results for an idealized high Tc-compound are compared to density functional (DF) calculations for the same system. It is shown that and why the DF-charge distribution is wrong. The largest deficiency arises for the Cu-d(x2-y2)-occupation, originating from strong atomic correlations but mostly from anomalous neighbor Cu-spin correlations. Both features are beyound the range of the homogeneous electron gas approximation underlying the DF-schemes. The ab-initio results also exclude a description of the real system in a Mott-Hubbard scenario, that is mostly chosen in theory.Comment: 8 pages, 1 figur

Electrons in High-Tc Compounds: Ab-Initio Correlation Results

Electronic correlations in the ground state of an idealized infinite-layer high-Tc compound are computed using the ab-initio method of local ansatz. Comparisons are made with the local-density approximation (LDA) results, and the correlation functions are analyzed in detail. These correlation functions are used to determine the effective atomic-interaction parameters for model Hamiltonians. On the resulting model, doping dependencies of the relevant correlations are investigated. Aside from the expected strong atomic correlations, particular spin correlations arise. The dominating contribution is a strong nearest neighbor correlation that is Stoner-enhanced due to the closeness of the ground state to the magnetic phase. This feature depends moderately on doping, and is absent in a single-band Hubbard model. Our calculated spin correlation function is in good qualitative agreement with that determined from the neutron scattering experiments for a metal.Comment: 21pp, 5fig, Phys. Rev. B (Oct. 98

Correlation energy contribution to cohesion in covalent structures

The contribution of electron correlations to the cohesive energy of covalent structures is discussed. Thereby a distinction is made between ab initio calculations done within a finite basis set of Gauss-type orbitals and simplified correlation calculations. By considering diamond, silicon and polyethylene in detail it is shown that the different correlation contributions have simple physical meanings.La contribution des corrélations électroniques à l'énergie de cohésion des structures covalentes est discutée. Par ce moyen la discussion est faite entre les calculs ab initio sur une base finie d'orbitales de type gaussien et les calculs simplifiés de corrélation. En considérant le diamant, le silicium et le polyéthylène en détail, on montre que les différentes contributions à l'énergie de corrélation ont une signification physique simple