Screening Diffusion of Small Molecules in Flexible Zeolitic Imidazolate Frameworks Using a DFT Parameterized Force Field

Quantitatively modeling adsorbate diffusion through zeolitic imidazolate frameworks (ZIFs) must account for the inherent flexibility of these materials. The lack of a transferable intramolecular ZIF force field (FF) for use in classical simulations has previously made an accurate simulation of adsorbate diffusion in many ZIFs impossible. We resolve this problem by introducing a density functional theory parameterized force field (FF) for ZIFs named the intraZIF-FF, which includes perturbations to the class I force fields previously used to model ZIFs. This FF outperforms ad hoc force fields at predicting ab initio relative energies and atomic forces taken from fully periodic ab initio molecular dynamics simulations of SALEM-2, ZIF-7, ZIF-8, and ZIF-90. We use the intraZIF-FF to predict the infinite dilution self-diffusion coefficients of 30 adsorbates with molecular diameters ranging from 2.66 to 7.0 angstrom in these 4 ZIFs. These results greatly increase the number of adsorbates for which accurate information about molecular diffusion in ZIFs is available.