Here it is interesting to note that the direct interaction of the adsorbed CO with the closest Zr surface ion does not only cause a change in charge state of this Zr ion, but affects also the other Zr ions surrounding the O-vacancy. For example, CO adsorption on the interface (i) site leads to a re-oxidation not only of the 2Zr ion from +2.34 e to +2.59 e, but also of the 1Zr ion (from +2.17 e to +2.46 e) and of the other two Zr ions surrounding this O-vacancy (Tables and) Hence, while the interaction between CO and Zr surface ion is considered to be highly local, polarization effects can also modify the charge on more distant ions.