The previous sections indicated a clear correlation between the charge on the Ru adsorption site, i.e., on the Ru atoms contributing to the Ru-CO bond, and the CO adsorption energy for CO adsorption on the interface sites. For adsorption on the top layer and middle layer sites, where due to the metallic screening the charge on the Ru atoms changes little upon O-vacancy formation, O-vacancy induced changes in the CO adsorption energy are generally very small.