Abstract
Today’s state-of-the-art method for calculating the interaction of atoms or small molecules with metal surfaces is considered to be density functional theory (DFT) at the generalized gradient approximation (GGA) level employing a slab or supercell representation of the surface. The method is widely used and by many assumed to be both qualitatively and quantitatively accurate. This notion has recently been challenged by Feibelman et al. [J. Phys. Chem. B 105, 4018 (2001)] who suggest that the DFT/GGA method does not correctly predict the most stable adsorption site for the CO/Pt(111) system, and they conclude that the method is not qualitatively accurate. However, using a different calculational approach we find a good agreement between the calculated potential energy surface for this system and the one inferred from experiments, indicating that the evidence supporting the view of Feibelman et al. is not yet conclusive. On the contrary, we advocate the view that the DFT/GGA method should at the moment be considered qualitatively accurate for predicting the most stable CO adsorption sites on metal surfaces. This view is supported by (i) our results for the Pt(111) surface which in agreement with experiments favors the top site, (ii) an assessment of literature results for other surfaces, suggesting that the error in the relative stability of the CO adsorption sites on a given surface is within ±0.1 eV when compared to experiments, (iii) the considerable challenge one faces when trying to converge DFT/GGA calculations within ±0.1 eV with respect to all computational parameters, (iv) and that for energy differences smaller than say 0.1 eV, calculated quantities like, e.g., vibrational frequencies and geometries discriminate correctly between sites, being in agreement with experiments at the correct adsorption site.
Highlights
The interaction of atoms and small molecules with metal surfaces is of great academic interest, but is extremely important in industry and society
Our results indicate that the DFT/GGA method correctly predicts the most stable adsorption site for this system
Taking all existing evidence together it seems that the DFT/GGA error falls within Ϯ0.1 eV for the relative stability of the CO adsorption sites on metal surfaces, and there are enough indications that it is difficult to converge DFT/ GGA calculations in all computational parametersreal space integration, k-space integration, number of layers in the slab/supercell, basis sets, spherical harmonic expansions, lattice sums, modelling of relativistic effects, pseudopotentials, etc.͒ to better than Ϯ0.1 eV. We think this provides support for the view that the DFT/GGA method should at the moment be considered qualitatively accurate for predicting the most stable CO adsorption sites on metal surfaces
Summary
The interaction of atoms and small molecules with metal surfaces is of great academic interest, but is extremely important in industry and society. The state-of-the-art method of today for calculating the interactions between atoms/molecules and metal surfaces is considered to be density functional theoryDFTat the generalized gradient approximationGGAlevel employing a slab or supercell representation of the surfacesee, e.g., Refs. One reason is that the results of a slab/supercell calculation prove rather easy to converge with respect to the number of layers usedsee, e.g., Refs. 12, 16 –21, but note that the very recent results of Ref. 22 indicate that it might be feasible Another reason is that the introduction of the GGAs ‘‘fixed’’ the wrong site-preference for CO adsorption on Cu100͒,5,12 and the lack of reaction barriers to dissociation for the H2 /Cu system[6,7,8] as calculated within the local density approximationLDAof DFT
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