We examine the adsorption of CH4 on the MgO(001) surface by a hybrid approach. It combines MP2 calculations with extrapolation to the complete basis set limit for the adsorption site and the CH4–CH4 pair interactions in the adsorbate layer, with DFT+dispersion calculations under periodic boundary conditions for the whole system. To the total binding energy of 10.7 kJ mol−1, the DFT+D(ispersion) correction contributes 0.7 kJ mol−1 only, showing that the Mg9O9 two-layer surface model is an excellent choice and that the interaction between the CH4 molecules in the adsorbate layer is dominated by pair interactions. Contributions due to relaxation of the atom positions of 0.6 kJ mol−1 (evaluated at DFT+dispersion) and of higher order correlation effects of 2.0 kJ mol−1 (evaluated by CCSD(T)) yield a final estimate of 13.3 kJ mol−1. To this total adsorption energy, the lateral interactions between the CH4 molecules in the adsorbate layer contribute substantially, 4.1 kJ mol−1.
“Observed” desorption energies of 15.3 and 16.0 kJ mol−1 have been derived from the observed Arrhenius desorption barriers (12.6 and 13.1 kJ mol−1) using thermal enthalpy contributions and a substantial zero-point energy (4.2 kJ mol−1) calculated from DFT+D vibrational frequencies. The comparison shows that our final hybrid MP2 : PBE+D+ΔCCSD(T) estimate has reached chemical accuracy. It misses 2–3 kJ mol−1 of binding only, which is most likely due to missing higher order correlation effects.
PBE+D(ispersion) itself yields an adsorption energy that agrees within 1 kJ mol−1 with our final hybrid MP2 : PBE+D+ΔCCSD(T) estimate.
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