MODEX: A Modular and Extensible Force Field for Metal-Organic Frameworks

The development of accurate and transferable force fields for metal-organic frameworks (MOFs) remains a significant challenge. Here, we introduce MODEX, a modular and extensible force field for MOFs. MODEX employs a building block approach in which the inorganometallic node and organic linker are parametrized separately and subsequently combined. Currently, MODEX is parametrized for ten representative Zr-based MOFs. Zr-based MOFs represent an important class of MOFs with a wide range of applications; however, limited transferable force fields have been developed exclusively for this family. The parametrization is driven by a custom multiobjective particle swarm optimization (PSO) algorithm implemented in our Force Field Factory program, enabling simultaneous fitting to multiple types of quantum chemistry reference data. The resulting force field accurately reproduces the structural, phonon, and mechanical properties of the ten target Zr-based MOFs. Furthermore, MODEX accurately captures the rotational dynamics of the linker in UiO-66, reproducing both the rotational barrier and the thermally accessible conformational space obtained from quantum chemistry calculations. MODEX is designed for continuous expansion; new building blocks can be readily parametrized using Force Field Factory. This work establishes MODEX as a robust force field for large-scale simulations of the ten Zr-based MOFs studied here and provides a platform for future extension to new frameworks.