This can again be explained by an efficient metallic screening. In contrast, there are significant changes in the CO adsorption energy for adsorption at the interface sites. For adsorption on the interface (i) site, where CO adsorption is not stable on the fully oxidized system, but stabilized by an O-vacancy on the 1i site (Figure ), we find little variation up to the 3VO system, but a distinct decrease of the CO adsorption energy for the 5VO system. Possible reasons for that will be discussed in more detail below.