  We construct an effective Hamiltonian for the motion of t2g highly correlated
states in NaxCoO2. This three-band model includes the indirect Co-O-Co hopping
t and the crystal-field splitting 3D. Calculations in a CoO6 cluster give the
effective parameters t=100 meV and 3D=315 meV. The Hamiltonian is solved using
a generalized slave-boson mean-field approximation. The results show a
significant band renormalization. Without any additional hypothesis, the
dispersion of the bands near the Fermi energy and Fermi surface topology agrees
with angle-resolved photoemission experiments, in contrast to predictions using
the local-density approximation.
