Ab initio derived force fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFs

In this paper we parameterized in a consistent way a new force field for a range of different zeolitic imidazolate framework systems (ZIF-8, ZIF-8(H), ZIF-8(Br) and ZIF- 8(Cl)), extending the MOF-FF parameterization methodology in two aspects. First, we implemented the possibility to use periodic reference data in order to prevent the difficulty of generating representative finite clusters. Second, a more efficient global optimizer based on the covariance matrix adaptation evolutionary strategy (CMA-ES) was employed during the parameterization process. We confirmed that CMA-ES, as a state-of-the-art black box optimizer for problems on continuous variables, is more suitable for force field optimization than the previous genetic algorithm. The obtained force field was then fully validated with respect to static and dynamic properties. Much effort was spent to ensure that the FF is able to describe the crucial linker swing effect in a large number of ZIF-8 derivatives. For this reason we compared our force field to ab initio molecular dynamic simulations and found an accuracy comparable to those obtained by different exchange–correlation functionals.