Next, we evaluated the change in the charge state of the ZrO2 support, of the Ru nanorod and of the CO molecule upon CO adsorption, which provides information on the charge transfer to the CO molecule (see Figure ). For adsorption on the interface (i) site, there are only negligible CO adsorption induced changes of the charge on the support, independent of the position of the O-vacancy. For the Ru nanorod, the change is more pronounced, around +0.2 e, independent of the position of the O-vacancy. Accordingly, we find a slightly bigger negative change in electron charge for the adsorbing CO. Also here, the variation is too small to have a sizable impact on the CO adsorption energy. Apparently, the significantly higher charge on the Ru adsorption site on the 1VO(1i) system has little effect on the charge transfer to the CO. In combination, these effects lead to rather similar CO adsorption energies on the middle layer and interface (i) sites, which is also independent of the presence of an O-vacancy and its position.