11. An Evaluation of Density Functional Theory for CO Adsorption on Pt(111)

In this study, serveal different density functionals were applied to study the CO adsorption on Pt(111) clusters. When adding more contribution of HF exchange energy, it can be found that the HOMO energy is decreased and LUMO energy is increased, thus increasing the HOMO-LUMO energy gap. The accuracy of S-T excitation energy can also be largely improved when increasing the ratio of HF exchange energy. For CO adsorption at Pt_7–3 cluster, most functionals predict that CO favors to adsorb at fcc site. Only when adding more than 40 % HF exchange energy, the M06HF, BMK, wB97 and M06-2X functionals can predict the top-site preference. For CO adsorption at Pt_9–9–9 cluster, when adding more than 40 % HF exchange energy, the CO prefers to adsorb at top site than fcc site. Among these functionals, the M06HF strongly predicts the top-site preference. The chemical bonding analysis shows that the effects of σ-repulsion are reduced as the CO S-T excitation energy increasing, and the effect of reduction for CO at top site is more remarkable than that for CO at fcc site. Therefore, CO would more favor to adsorb at top site in those functionals which can give better CO S-T excitation energy. Although the opposite trend can be found for the π-attraction, the overall effect also supports CO favoring to adsorb at top site.