  CO adsorption on the Pt(111) surface is studied using first-principles
methods. As found in a recent study [Feibelman, et al., J. Phys. Chem. B 105,
4018 (2001)], we find the preferred adsorption site within density functional
theory to be the hollow site, whereas experimentally it is found that the top
site is preferred. The influence of pseudopotential and exchange-correlation
functional error on the CO binding energy and site preference was carefully
investigated. We also compare the site preference energy of CO on Pt(111) with
the reaction energy of formaldehyde formation from H$_2$ and CO. We show that
the discrepancies between the experimental and theoretical results are due to
the generalized gradient approximation (GGA) treating different bond orders
with varying accuracy. As a result, GGA results will contain significant error
whenever bonds of different bond order are broken and formed.
