First-Principles-Derived Force Field for Copper Paddle-Wheel-Based Metal-Organic Frameworks

We present a fully flexible and ab initio-derived molecular mechanics force field for the ubiquitous copper paddle-wheel building block Cu-2(O2C)(4) in metal-organic frameworks. The force field expression is based on the established MM3 force field, extended by additional cross terms and specific bond-stretching and angle-bending terms for the square-planar CuO4 coordination environment. Using reference data computed at the DFT level for nonperiodic reference systems, the parametrization is performed using an automated genetic algorithm optimization strategy in order to reproduce structure and low normal modes of the model systems. It is validated on the much investigated Cu-btc (HKUST-1) metal-organic framework. Beyond the structure, lattice-dynamic-dependent properties such as the bulk modulus and the observed negative thermal expansion effect of Cu-btc are quantitatively predicted by the force field without recourse with respect to experimental data. In connection with available parametrizations of various organic linkers, it can be useful for aiding the structure determination of known MOFs, but it also paves the way for the computational prescreening of yet unknown copper paddle-wheel-based frameworks.