Adsorption of CH₄ on the Pt(111) surface: Random phase approximation compared to density functional theory

We investigate the adsorption of CH₄ on the Pt(111) surface for two adsorption modes, hcp (hexagonal closed packed) hollow tripod and top monopod in a (√3 × √3)R30° surface cell that corresponds to experimental surface coverage. Surface structures are optimized with density functional theory using the Perdew-Burke-Ernzerhof (PBE) functional augmented with the many-body dispersion scheme of Tkatchenko (PBE+MBD). Whereas the Random Phase Approximation (RPA) predicts a clear preference of about 5 kJ mol^-1 for the hcp tripod compared to the top monopod structure, in agreement with vibrational spectra, PBE+MBD predicts about equal stability for the two adsorption structures. For the hcp tripod, RPA yields an adsorption energy of -14.5 kJ mol^-1, which is converged to within 1.0 ± 0.5 kJ mol^-1 with respect to the plane wave energy cutoff (500 eV), the k-point mesh (4 × 4 × 1), the vacuum layer (about 10.3 Å, with extrapolation to infinite distance), and the number of Pt layers (3). Increments for increasing the number of Pt layers to 4 (+1.6 kJ mol^-1) and the k-point mesh to 6 × 6 × 1 (-0.6 kJ mol^-1) yield a final estimate of -13.5 ± 2.1 kJ mol^-1, which agrees to within 2.2 ± 2.1 kJ mol^-1 with experiment (-15.7 ± 1.6), well within the chemical accuracy range.