For adsorption on the interface (i) site, the slight up-shift of the frequency with increasing distance between O-vacancy and CO adsorption site, from the 1i to the 4i site (see left panel, Figure ), which was observed for individual O-vacancies, changes into a more pronounced down-shift for the growing O-vacancy cluster, from the 1VO to the 2VO and in particular the 3VO system (see right panel, Figure ). This goes along with a considerable increase of the electron charge on the Ru adsorption site (Figure ) and a slight decrease/increase of the charge transfer to the adsorbed CO for the 2VO/3VO system (Figure ). The latter trend agrees with decrease in C-O vibrational frequency upon increasing charge transfer to the CO. Similar agreement in trends is also obtained upon formation of the 4 th and 5 th vacancies (5VO system), where a slightly increasing charge C-O frequency correlates with a slightly decreasing charge transfer. ˗ For adsorption on top layer sites, these latter effects are negligible, except for coordination effects, and adsorption resembles that on metal surfaces. For adsorption on interface sites, these effects can be substantial, depending on the position of the O-vacancy. The systematic difference in CO adsorption energy between interface (i) and interface (ii) sites is mainly due to higher positive contributions from COad induced deformation energies and changes in interface energy. For adsorption on the middle layer Ru-5a site, its higher adsorption energy is mainly attributed to coordination effects. Contributions from charge transfer to the Ru adsorption site, or deformation/interface energies are generally small, and except for a single case direct interactions between CO and a partly reduced Zr surface ion do not play an important role.