For adsorption on the interface sites, where CO binds to Ru atoms in the bottom layer (interface layer), whose charge state varies significantly with the position of the O-vacancies and the size of O-vacancy clusters, the variations of the CO adsorption energy are more significant. To test for a general correlation between charge state of the Ru adsorption site and the CO adsorption energy, we plotted the CO adsorption energy on the interface sites as a function of the charge state of the Ru adsorption site. Figure illustrates the correlation between these properties for various different O-vacancy positions and O-vacancy cluster sizes for CO adsorption on the interface (i) site (Figure ) and on the interface (ii) site (Figure ). In these plots we did not consider those cases where as discussed in sections 3.2.1 -3. ) and finally the actual CO binding energy on the relaxed adsorption site (𝐸 𝑏𝑖𝑛𝑑,𝐶𝑂 ), the lower CO adsorption energy for CO adsorption on a Ru/ZrO2-x system with a 5VO vacancy cluster underneath the Ru nanorod is partly due to a significant (positive) contribution from the sum of the COad induced deformation energies and change in interaction energies, which is around 0.4 eV, which leads to a lowering of the CO adsorption energy (Table ). In addition, also the CO binding energy is lower than in other cases.