Evaluation of ZIF-8 flexible force fields for structural and mechanical properties

Abstract

Metal–Organic Frameworks (MOFs) offer considerable potential for applications in adsorption due to their large pore volumes and surface areas. Studies on mechanical stability of MOFs are scarce. Seminal experimental work has shed a new light on the role that elastic constants play in establishing the structural stability of the prototypical ZIF-8 MOF, with its elastic deformation mechanism being linked to the pliant ZnN4 tetrahedra (Tan et al., 2012). Over the past decade several classical flexible force fields have been proposed to study the physical properties of the system using simulations (Zheng et al., 2012; Hu et al., 2012; Zhang et al., 2013, Wu et al., 2014; Krokidas et al., 2015; Weng and Schmidt, 2019; Dürholt et al. 2019). In this work, we evaluated the majority of them for reproducing structural and mechanical properties (unit cell sizes as a function of temperature and pressure, and elastic constants as a function of pressure), compared them to existing DFT calculations (Tan et al., 2012; Maul et al., 2019) and found that they provide different results under the same testing conditions. The obtained results provide insight into the relationship between fundamental elastic properties and the chosen force field parametrization, allowing us to characterize the applicability of each of the force fields. Furthermore, the employed two-code approach allowed us to find significant discrepancies in elastic constant values for the same force field between methodologies that employ different energy minimization algorithms, suggesting that eigenmode-following approaches might be needed to guarantee true minimum energy configurations for ZIFs.