Hence, a decreasing electron density at the adsorption site disfavors electron transfer from the ), in full agreement with the trend in the CO adsorption energy. For the interface (i) site, we obtain a continuous increase of the d-band center up to the 3VO system (black line in Figure , left), in qualitative agreement with the trend in the CO adsorption energy (Figure ). For the 5VO energy compared to that on the 2VO system. The calculated CO adsorption energy indeed decreases from the 2VO to the 3VO system, but the change is very small (Figure ). Finally, for the 5VO system, the d-band center remains about constant, while the CO adsorption energy decreases significantly (Figure ). Considering that also the interaction between CO and the 2Zr surface ion changes very little, as indicated by the about constant distance (Table ), this illustrates the limits of a quantitative correlation between d-band center energy, in particular on more complex systems. Comparison with the trends in the different contributions to the CO adsorption energy does (Figure ) shows negligible changes in the deformation and interface energies when going from the 1VO to the 3VO system, which remain also for the 5VO system, leaving the CO adsorption energy almost constant. Apparently, this change in deformation and interface energy is related to a COad induced re-positioning of the Ru-4a atom by about 0.5 A (d max Ru in Table ), as a consequence of a strong interaction between COad and the Ru-4a atom, which is responsible also for the change in adsorption site from Ru-1a/Ru-5a to the Ru1a/Ru-4a/Ru-5a site. Addition of two more O vacancies in the 5VO system causes a decrease of the CO binding energy, which is lower than in other cases. Together with the positive contributions (about 0.4 eV) from the deformation energies plus the difference in interaction energies, this leads to a significant lowering of the CO adsorption energy (Figure and Table ).