Ab initio evaluation of the local effective interactions in the superconducting compound Na0.35CoO2•1.3H2O

We used ab initio quantum chemical methods, treating explicitly the strong correlation effects within the cobalt 3d shell, as well as the screening effects on the effective integrals, for accurately determining on-site and nearest-neighbor (NN) interactions in the Na0.35CoO2 center dot 1.3H(2)O superconducting compound. The effective ligand field splitting within the t(2g) orbitals was found to be delta similar to 300 meV, the a(1g) orbital being destabilized compared to the e(g)(') ones. The effective Hund's exchange and Coulomb repulsion were evaluated to J(H)similar to 280 meV and U similar to 4.1-4.8 eV for the a(1g) orbitals. The NN hopping parameters were determined within the three t(2g) orbitals and found to be of the same order of magnitude as the t(2g) ligand field splitting. This result supports the hypothesis that a three band model would be better suited than a one-band model for this system. Finally we evaluated the NN effective exchange integral to be antiferromagnetic and J=-66 meV.