Electronic structure and Fermi-surface topology of NaxCoO2

We construct an effective Hamiltonian for the motion of T-2g highly correlated states in NaxCoO2. We solve exactly a multiband model in a CoO6 cluster with electronic occupation corresponding to a nominal Co valence of either +3 or +4. Using the ensuing ground states, we calculate the effective O mediated hopping t=0.10 eV between many-body T-2g states and estimate the direct hopping t(')similar to 0.05 eV. The trigonal splitting 3D=0.315 eV is taken from recent quantum chemistry calculations. The resulting effective Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization and a Fermi-surface topology that agrees with experiment, in contrast to predictions using the local-density approximation.