Reactive Coarse Grained Force Field for Metal-Organic Frameworks applied to Modeling ZIF-8 Self-Assembly

Abstract

Decoding the self-assembly mechanism of metal-organic frameworks is a crucial step in reducing trial-and-error tests in their synthesis protocols. Atomistic simulations have proven essential in revealing molecular-level features of MOF nucleation, but they still exhibit limitations in the simulation setups due to size constraints (inability of reaching realistic concentrations or exploring non-stoichiometric metal:ligand ratios). In this contribution, we develop a methodology to derive reactive coarse grained force fields based on multiscale coarse graining methods. We apply our novel methodology to the case of the archetypal zeolitic-imidazolate framework ZIF-8. Our coarse grained force field, which we call nb-CG-ZIF-FF, does not contain any explicit connectivity information, but learns the tetrahedral Zn-connectivity from many body correlations within an atomistic benchmark. nb-CG-ZIF-FF quantitatively reproduces the features of bulk, crystalline ZIF-8 as well as the structural evolution of pre-nucleation species in terms of Zn n-fold coordination populations from the atomistic benchmark. While the range of rings that are formed along the synthesis process are well captured by nb-CG-ZIF-FF, the model cannot exactly reproduce ring populations. Our reactive CG force field fitting approach can be applied to any MOF, opening new research avenues in modeling MOF formation, decomposition, defect dynamics and phase transition processes.