The lower CO adsorption energies on the interface (ii) sites compared to interface (i) sites are again mainly due to larger combined (positive) contributions from COad induced deformation energies and changes in interface energies. Apart from these effects, the trend in CO adsorption energy follows at least qualitatively the trends in the charge and in the d-band center on the respective adsorption sites CO before adsorption. Finally, interactions between CO and partly reduced Zr surface ions adjacent to O-vacancies, can significantly increase the CO adsorption energy. Such interactions, which are indicated by close distances between CO and the respective Zr surface ion, are not captured by the d-band model, and the same is true also for COad induced deformation energies and changes in the interface energies. Nevertheless, when focusing on a specific adsorption site, the correlation between CO adsorption and charge / energy of the d-band center on the adsorption site works rather well (see also section 3.2.5), underlining the potential of using the charge or d-band center position before CO adsorption as descriptor for the CO adsorption energy also in such complex systems as the present ones.