Furthermore, CO adsorbed on interface (i) and (ii) sites can interact also with the 2Zr and 1Zr cations, respectively. These and other changes in the initial adsorption site are indicated in the following figures by hatched bars for the CO adsorption energy. The energetically preferred adsorption sites on the different systems are listed in Tables and, respectively, and also mentioned in the discussion. It should also be noted that in some cases CO was not (meta-) stable on the initial adsorption site and relaxed to another site, e.g., from an interface (i) site to a middle layer site. This would be indicative of a very small or absent energy barrier for CO migration between the two configurations.