Optical Properties and Correlation Effects in NaxCoO2
  We have calculated the optical spectra of Na$_{x}$CoO$_2$ for $x$=0.3, 0.5, and 0.7 within the LDA. We compare our results to available experimental data and show that the important features and trends are reproduced well, but there is a nearly uniform shift of peak positions and poor agreement in intensities. We show, through application of a simple model, that these differences can be attributed to overhybridization between Co and O orbitals and spin fluctuations which renormalize the bandwidth. Applying the LDA+U procedure shifts the optical peaks further from their experimental locations, indicating that this method of incorporating correlation effects is ill-suited for the case NaxCoO2. 
M.D. Johannes, I.I. Mazin, D.J. Singh