Challenges in Molecular Dynamics of Amorphous ZIFs using Reactive Force Fields

While amorphous metal–organic frameworks (a-MOFs) form an emerging class of materials of growing interest, their structural characterization remains experimentally and computationally challenging challenging. Out of the many molecular simulation methods that exist to model these disordered materials, one strategy consists in simulating the phase transition from a crystalline MOF to the amorphous state using molecular dynamics. ReaxFF reactive force fields have been proposed for this purpose in several studies to generate models of zeolitic imidazolate frameworks (ZIFs) glasses by melt-quenching. In this work, we investigate in detail the accuracy and reliability of this approach by reproducing the published procedures and comparing the structure of the resulting glasses to other data, including ab initio modeling. We find that the in silico melt-quench procedure is extremely sensitive to the choice of methodology and parameters, and suggest adaptations to improve the scheme. We also show that the glass models generated with ReaxFF are markedly different from their ab initio counterparts, as well as known experimental characteristics, and feature an unphysical description of the local coordination environment, which in term affects the medium-range and bulk properties.