Efficient Calculation of Small Molecule Binding in Metal-Organic Frameworks and Porous Organic Cages

The activation, storage, and separation of gases and fuels are closely related to the reduction of greenhouse gas emissions, the widespread use of renewable energies, and the application of industrial gases. Metal-organic frameworks (MOF) and porous organic cages (POC) are an emerging class of crystalline porous materials that show promising characteristics in this field. Yet, their accurate theoretical description poses a challenge to existing methods due to the sheer size of the pores and cages as well as their often complex structure. In this work, the performance of generally applicable density functional approximations (DFAs), semiempirical quantum mechanical (SQM) methods, and force fields (FFs) for the calculation of binding energies of various gases in molecular cutouts of MOFs and POCs is tested with reference to high-level PBE0-D4/def2-TZVP hybrid DFT energies. Therefore, favorable binding sites for greenhouse gases (CO2), energy-related gases (H-2, methanol, and benzene), and industrial gases (N-2) are determined by an efficient conformer search algorithm (CREST). The resulting structures are further optimized by DFT (B97-3c), semiempirical (GFN2-xTB), and force-field (GFN-FF) methods to yield the binding sites and corresponding energies. With mean absolute deviations ranging from 1.1 to 1.4 kcal mol(-1) for all tested systems, the considered GFN methods reach an accuracy remarkably close to the DFT reference, justifying their application for efficient binding site screening. In comparison, the widely used PMx methods show on average 1.0 kcal mol(-1) larger deviations. Furthermore, the application of single-point, multilevel approaches and the parallelism of potential energy surfaces are discussed.