  We scrutinize the accuracy of the pseudopotential approximation in
density-functional theory (DFT) calculations of surfaces by systematically
comparing to results obtained within a full-potential setup. As model system we
choose the CO oxidation at a RuO2(110) surface and focus in particular on the
adsorbate binding energies and reaction barriers as target quantities for the
comparison. Rather surprisingly, the major reason for discrepancy does not
result from the neglected semi-core state relaxation in the frozen-core
approximation, but from an inadequate description of the local part of the Ru
pseudopotential, responsible for the scattering of f like waves. Tiny,
seemingly irrelevant, imprecisions appearing in these properties can have a
noticeable influence on the surface energetics. At least for the present
example, we obtain excellent agreement between both approaches, if the
pseudopotential describes these scattering properties accurately.
