An ab initio evaluation of the local effective interactions in the NaxCoO2 family

We used quantum chemical ab initio methods to determine the effective parameters of Hubbard and t-J models for the Na(x)CoO(2) compounds (x = 0 and 0.5). As for the superconducting compound we found the a(1g) cobalt orbitals above the e'(g) ones by a few hundreds of meV due to the e'(g)-e(g) hybridization of the cobalt 3d orbitals. The correlation strength was found to increase with the sodium content x, whereas the in-plane AFM coupling decreases. The less correlated system was found to be the pure CoO(2), however it is still strongly correlated and very close to the Mott transition. Indeed we found U/t ∼ 15, which is the critical value for the Mott transition in a triangular lattice. Finally, we found the magnetic exchanges in the CoO(2) layers to be strongly dependent on the weak local structural distortions.