Figure illustrates the total charge of the Ru atoms binding to CO at the interface sites. For adsorption on the interface (i) site (Ru-1a, Ru-5a, 2Zr) of the fully oxidized Ru/ZrO2 system, where CO adsorption is not stable, the charge on this site would be rather small, around 0.1 e (Table ), which may provide a simple explanation for the spontaneous shift of COad from this site to the middle layer site (Ru-5a) during relaxation. In the presence of one or two Ovacancies, this charge increases from -0.59 e (1VO) to -0.92 e (2VO), which is mostly due to a pronounced charge increase on the Ru-1a atom in the bottom layer, from -0.62 e (1VO) to -1.02 e (2VO) (Table ). For the 3VO system, COad moves to the threefold hollow site formed by the Ru-1a, Ru-4a and Ru-5a atoms (Table ). Because of the little charge on the Ru-4a atom and the small charge changes on the other two Ru atoms (Table ), formation of the third Ovacancy (3VO) has little effect on the charge on the adsorption site (-0.96 e). Finally, upon formation of the fourth and fifth vacancy, for the 5VO system, the charge on the Ru adsorption site decreased abruptly to -0.13 e. Also this is mainly due to a change in the Ru adsorption site, since the more strongly charged Ru-1a atom, with its significantly enhanced Ru-1a -C(CO) bond distance (2.29 Å, Table ) is not considered as part of the adsorption site in the most stable configuration any more. On the other hand, however, based on the distance between Ru-1a and CO, there still seems to be some interaction between CO and the Ru-1a atom in the 5VO system. Therefore, we additionally plotted a data point which includes also the charge on the Ru-1a atom for comparison. Assuming that the real situation will be somewhere between the two data points for the 5VO system, there is some decrease in the charge state, by about 0.3 e to lower charge, which brings it back to the value obtained for the 1VO system. Considering this, the change in charge state from 3VO to 5VO seems to be much less than the change in CO adsorption energy. Finally, it is interesting to note that this decay in adsorption energy is not reflected by a change in electron charge on the support, Ru nanorod or CO upon CO adsorption (Figure , left). We will get back to this open question when discussing the shifts in the energy of the d-band center further below.