  The electronic state in a CoO_2 plane of the layered cobalt oxides
Na_{x}CoO_2 is investigated by using the 11 band d-p model on a two-dimensional
triangular lattice, where the tight-binding parameters are determined so as to
fit the LDA band structure. Effects of the Coulomb interaction at a Co site:
the intra- and inter-orbital direct terms U and U', the exchange coupling J and
the pair-transfer J', are treated within the Hartree-Fock approximation. We
also consider the effect of the Na order at x=0.5, where Na ions form one
dimensional chains, by taking into account of an effective one-dimensional
potential Delta epsilon_{d} on the CoO_2 plane. It is found that the Na order
enhances the Fermi surface nesting resulting in the antiferromagnetism (AFM)
which is suppressed due to the frustration effect in the case without the Na
order. When U and Delta epsilon_{d} are varied, we observe three types of the
AFM: (1) the metallic AFM with large density of states N_F at the Fermi level
for small values of U and Delta epsilon_{d}, (2) the semimetallic AFM with tiny
N_F for large U with small Delta epsilon_{d} and (3) the insulating AFM with a
finite energy gap for large values of U and Delta epsilon_{d}.
