For CO adsorption on the interface (ii) site, we also find a linear correlation between charge on the adsorption site and CO adsorption energy. Also for this fit, we did not include the cases with significant contributions from direct interactions between CO and partly reduced Zr surface ions or COad induced migration of the O-vacancy (see discussion in sections 3.2.1 -3.2.3). Interestingly, for adsorption on this site the slope of the fit line is much steeper than obtained for adsorption on the interface (i) site. Apparently, CO adsorption on this site is much more sensitive to changes in the charge on the Ru adsorption site than adsorption on the interface (i) site. We will get back to this in a moment.