DFT-based force field development for noble gas adsorption in metal organic frameworks

Density Functional Theory (DFT) based force fields (FFs) for Ar and Xe adsorption in six metal-organic frameworks were developed using three DFT functionals (PBE-D2, vdW-DF, vdW-DF2) in periodic systems. These force fields include van der Waals (vdW) and polarization terms, and the effect of the latter was shown to be small. Using our DFT-derived and standard (UFF) FFs in grand canonical Monte Carlo simulations, adsorption isotherms and heats of adsorption were calculated and compared with experiment. In most of the cases, it was possible to accurately predict adsorption isotherms using one of the DFT-derived FFs. Still, among the DFT functionals investigated, no single DFT functional could accurately describe all of the adsorbate-framework interactions. On average, performance of UFF and PBE-D2 based FFs to predict experimental isotherms were at a similar quality, still, UFF was slightly superior. Although vdW-DF2 based FFs predicted experimental isotherms almost perfectly for ZIF-8 and HKUST-1 up to 20 bar, their average performance was less than that of PBE-D2 based FFs. Nevertheless, the overall performance of UFF, PBE-D2 and vDW-DF2 FFs was similar. Lastly, vdW-DF based FFs always over-predicted experiments.