Ab initio Free Energy Calculations with Chemical Accuracy for Molecule-Surface Interactions

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Atomistic understanding of heterogeneous catalysis as well as of storage and separation of energy-relevant gases in nanoporous materials requires the quantum mechanical ab initio prediction of free energies for large periodic systems with chemical accuracy (4 kJ/mol). We have shown that this can be achieved by (i) use of hybrid high level - low level calculations combining accurate wave-function methods for the reaction site with less accurate, but computationally more efficient density functional theory calculations on the full (pseudo) periodic system, and (ii) calculating anharmonic vibrational energies in an affordable way to improve the sampling of the potential energy surface. Such a method did not exist so far. It is proposed (1) to develop a robust and broadly applicable methodology that will be freely available to the community, taking the risk that the approximations we apply to calculate anharmonicities are not valid for all types of surface bonding and that a crucial technical step - curvilinear representation of normal modes - cannot be made sufficiently automatic, (2) to build a data base of molecule - surface interactions for which agreement is achieved with experimental enthalpies and/or entropies and which are, thus, suitable for benchmarking approximate methods, and (3) to use this methodology to obtain kinetic and thermodynamic data for elementary steps in selected applications, e.g., catalysis by zeolites as well as gas storage and separation by metal-organic frameworks.