To test for a correlation between the charge on the Ru adsorption site and the CO adsorption energy, we plotted the charge on the Ru atoms interacting with CO (before CO adsorption) in Figure . The charge on the Ru adsorption site (Ru-1a and Ru-5a atoms) is rather low for the fully oxidized system (0VO), which may explain the instability of CO adsorption on the interface (i) site under these conditions. It drastically increases upon formation of the O-vacancy at the 1i site, respectively, and then decreases again for the more distant O-vacancies (see also Table ). Interestingly, despite the significant modification of the charge on the Ru adsorption site upon O-vacancy formation on the 1i site, this is not reflected by a similarly significant change in the CO adsorption energy, underlining again that the charge on the Ru adsorption site is an important, but not the only factor affecting the CO adsorption energy and /or that the variation in adsorption energy with charge is only small (see also section 3.2.5).