Since there is no significant charge transfer to the top layer or middle layer sites, such variations are only possible on interface sites. The high adsorption energy on the Ru-5a middle layer site as compared to the top layer sites is again attributed to coordination effects, which play a role also for adsorption on the interface sites. The generally lower adsorption energy on the interface (ii) site is largely determined by a higher contribution from COad induced deformation energies and changes in interface energies, as compared to adsorption on the other sites. Finally, mainly for adsorption on interface sites, interactions between a Zr surface ion and the adsorbed CO molecule can play an important role, if this ion is activated by a neighboring O-vacancy, either a single O-vacancy or a vacancy in an O-vacancy cluster.