The adsorption of CO on Mg(001) constitutes a challenge for current density functional approximations because of its weak interaction character. In the present work we show that the M06-2X and M06-HF exchange-correlation functionals are the first ones to provide a simultaneously satisfactory description of adsorbate geometry, vibrational frequency shift, and adsorption energy of CO on MgO(001). For a sufficiently large embedded cluster model, the three functionals of the M06 family-which contain a nonzero percentage of Hartree-Fock exchange (M06, M06-2X, and M06-HF)-all predict positive C-O vibrational shifts, in agreement with the experimental findings, while the local M06-L functional gives large negative shifts. Moreover, the shifts computed with the M06-2X and M06-HF potentials are in good agreement with the experimental shift of +14 cm-1. The interaction energy (De) calculated with M06-2X and M06-HF is ∼6.0 kcal/mol, which agrees well with the De value (∼4 kcal/mol) deduced from the D0 obtained in thermal desorption measurements on single-crystal surfaces.
Bibliographical noteFunding Information:
Financial support was provided by the Spanish Ministry of Education and Science (Grant Nos. CTQ2005-08459-CO2-01, UNBA05-33-001, and CTQ2005-03222) and by Generalitat de Catalunya 2005SGR-00697 grant. This work was supported in part by the National Science Foundation under Grant No. CHE-0704974.
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