In total, these results indicate that also for O-vacancy clusters the changes in CO adsorption energy with increasing cluster size are generally small,  0.2 eV, when not considering the cases where additional direct CO -Zr interactions result in large changes of the adsorption energy. This is again significantly less than the difference between CO adsorption on top layer and interface (ii) sites on the one hand and middle layer sites and interface (i) sites on the other hand.