Similar calculations were performed for CO adsorption on the interface (ii) site. For the original structure of the model system, we obtain the trend in the d-band center illustrated in Figure , right panel. For the first three systems, there is little difference compared to the d-band center energies of the interface (i) site, while there are considerable differences in the CO adsorption energies on these sites. A possible reason for this discrepancy will be discussed below in this section. Second, there are only small changes in the d-band center for O-vacancies at the 1i and the more distant 4i site compared to the fully oxidized state, which also agrees well with the trend in the CO adsorption energy. For the O-vacancy on the 7i site, the d-band center suddenly up-shifts. This shift is mainly caused by the change in the Ru adsorption site, from Ru-2a / Ru-6a to Ru-2a / Ru-3a (Table ), since for this O-vacancy position the d-band center on the Ru-3a atom shows a pronounced upshift, while on the Ru-5a atom it down-shifts by 0.3 eV. These counteracting shifts provide a clear driving force for the spontaneous change in adsorption site