Significance of single-electron energies for the description of CO on Pt(111)

Semilocal density functionals predict that the stable adsorption site of carbon monoxide (CO) on Pt(111) is the hollow fcc site, in disagreement with experimental studies which indicate that CO adsorbs on the top site at low coverage. This site preference depends on a subtle balance between the interaction of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) with the metal substrate. Local and semilocal functionals seem to overestimate the interaction of the LUMO with the metal substrate, in turn favoring the wrong site. It is argued that this error is related to the ``gap'' problem of present density functionals and might be cured by functionals that increase the HOMO-LUMO separation.