The field of flexible metal–organic frameworks (MOFs) has opened an era of intriguing applications. This chapter gives a status on the theoretical understanding and insights in flexible MOFs. First, a classification of various types of flexibility is given encompassing local and spatially extended phenomena, and transformations maintaining bond integrity of inducing bond breaking. Next, the key elements of a modelling exercise are given, which include building structural models for MOFs representative for realistic materials, a theoretical description of the potential energy surface with quantum mechanical, force field, machine learning potentials or coarse grained based methods, deriving macroscopic thermodynamic and kinetic observables based on statistical physics and thermodynamics. Then, various representative modelling studies on prototypical flexible MOFs are discussed highlighting transformations triggered by various stimuli, inducing various flexibility modes, and connecting them with experiment. The chapter ends with the formulation of some perspectives related to the ambition to model flexibility in realistic MOFs on longer length and time scales, including spatial heterogeneities and being composed of elements for which advanced electronic structure methods are necessary. Further advances will benefit from synergies between theoreticians experimentalists and scientists from other fields like quantum physics, data science, spectroscopy, and imaging.
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