  The electronic band structures of the parent compound Na0.3CoO2 and the
hydrated superconductor Na0.3CoO2.1.3H2O were evaluated using the local spin
density approximation (LSDA). Systematic analysis on the bands near the Fermi
surface (FS) revealed that the hydration-induced modifications appear, notably
in the valence charge density and the orbital hybridization. The most striking
changes are the decrease of density in Cobalt's a1g state and the
rehybridization between O 2p and Co a1g in the hydrated superconducting phase.
These hydration-induced modifications could yield visible differences in
interband excitations and dielectric functions. Experimental results of
electron energy-loss spectroscopy (EELS) are fundamentally in agreement with
the theoretical predictions for the hydrated Na0.3CoO2.yH2O.
