Ab initio calculations with chemical accuracy for molecule – surface interactions
Joachim Sauer
Institut für Chemie, Humboldt-Universität, Unter den Linden 6, 10000 Berlin, Germany js@chemie.hu-berlin.de
The ab initio prediction of molecule-surface interaction energies with hundreds of atoms with an accuracy that is comparable to experiment is a challenging problem of computational quantum chemistry. We apply a hybrid method that uses MP2 at the reaction site and DFT-dispersion for the full periodic structures. We perform structure optimization at this level, and – as single point calculation – perform CCSD(T) calculations for smaller cluster models.
With this hybrid MP2:DFT-D + DCC approach we calculate adsorption energies for ten systems for which reliable experimental data were available:
(i) adsorption of CO, as well as CH4 and C2H6 on the MgO(001) surface,
(ii) adsorption of H2, CO, CO2 and CH4 in metal-organic frameworks (MOF),
(iii) adsorption of small alkanes in zeolite H-Chabazite. We show that the results agree within chemical accuracy limits with “experimental” reference energies. The latter are derived from measured enthalpies using zero-point vibrational energies and thermal energies obtained with PBE+D.
∆E (Ref) = ∆HT (Exp) + RT – ∆Etherm (DFT-D) – ∆EZPV(DFT-D),
We use these “experimental” reference energies to test different ways of taking dispersion into account within DFT as implemented in the VASP code:
(i) Parametrized 1/r6 terms “D2” and “D3” after Grimme, and after Tkatchenko/ Scheffler “TS”
(ii) Many body approach of Tkatchenko, including “HI” and “FI” variants
(iii) van der Waals functionals, vdW-revPBE and vdWoptB86b
The best performing ones were PBE+D2, PBE+MBD/HI and PBE+MBD/FI with mean unsigned errors of 6.5, 6.0 and 5.7 kJ/mol, respectively.