The charge transfer upon CO adsorption on the interface (ii) site is initially small, both for the fully oxidized system and 1VO system with the O-vacancy at the 1i site. Formation of the (ii) vacancy in the 2VO system results in a distinct re-oxidation of the support, in particular of the 1Zr and 2Zr surface ions (Tables and), but also of the other Zr ions around this vacancy, if these were partly reduced upon formation of the O-vacancy. Additional formation of the deeper lying (iii) and (iv) O-vacancies does not change this situation any more, which is in full agreement with the little changes in the CO adsorption energy and in the charge on the adsorption site for these systems. Finally, the tendency towards higher CO adsorption energies for the 2VO and larger clusters (Figure ) partly results from an increase in CO binding energy, but in this case contributions from the interface and deformation energies also play a role (Figure ).