CO adsorption on the interface (ii) site is less stable than on the interface (i) site, more comparable to that on the top layer sites. In the presence of an O-vacancy, we find a subtle increase of the CO adsorption energy, which remains about constant with increasing distance of the O-vacancy from the adsorption site, from -1.84 eV (fully oxidized) to -1.98 --2.05 eV.