With further increasing O-vacancy cluster size, the CO adsorption energy decreases slightly, but the decay is much less than that observed for adsorption on the interface (i) site when going to the 5VO system. For the 3VO system, this goes along with an essentially constant total charge on the Ru adsorption site and a slightly smaller electron transfer to the CO molecule upon CO adsorption. Finally, when moving from the 3VO to the 5VO system, we find a significant decrease in charge on the Ru adsorption site, mainly on the Ru-2a atom (Table ), while there is only a minor decay in the charge transfer to the adsorbing CO, and also the decay in CO adsorption energy is less pronounced than observed for the interface (i) site at this point (Figure and). Also this will be dealt with further when discussing the related shift in the d-band center.