A Tight-Binding Investigation of the NaxCoO2 Fermi Surface
  We perform an orthogonal basis tight binding fit to an LAPW calculation of paramagnetic Na$_x$CoO$_2$ for several dopings. The optimal position of the apical oxygen at each doping is resolved, revealing a non-trivial dependence of the band structure and Fermi surface on oxygen height. We find that the small e$_{g'}$ hole pockets are preserved throughout all investigated dopings and discuss some possible reasons for the lack of experimental evidence for these Fermi sheets. 
M.D. Johannes, D.A. Papaconstantopoulos, D.J. Singh, M.J. Mehl