Beyond direct molecular dynamics and thermodynamic integration, it is possible to use more advanced techniques in order to obtain free energy profiles, such as through the use of carefully-designed collective variables or through umbrella sampling or metadynamics. However, we stress that all the simulation methods that describe the coupling of adsorption and framework deformation, regardless of their strategy for the exploration of phase space, rely on an accurate description of the intermolecular interactions in the system, i.e., the use of accurate force fields. Moreover, these force fields must suitably describe the delicate balance of very different interactions, including the intramolecular flexibility, the deformation of the metal-organic coordination environment, and the host-guest interactions. Different strategies have been used for this, including ab initio (or first-principles) MD based on DFTlevel evaluation of the forces at each MD step; however its very high computational cost prevents the study of large systems or long simulations. Classical "universal" force fields, on the other hand, generally lack the necessary accuracy for a realistic chemical description of the framework materials. To address these long-standing limitations, many recent studies have focused on the systematic development of ad hoc force fields for specific {material, adsorbate} systems. Based on reference data obtained at the ab initio level -typically through DFT calculations -these force fields aim to have chemical accuracy with the computational expense of a classical force field. Dürholt et al. proposed, for example, an approach for the systematic optimization of force field parameters for flexible ZIFs based on near-equilibrium DFT data (relaxed structure, Hessian matrix, and elastic tensor). Ghanavati et al. recently followed a similar approach, using LASSO regression to prioritize the important features among the many intramolecular flexibility terms of an analytical potential, and training on a dataset that included configurations farther from equilibrium -dihedral scan of rotatable bonds, short ab initio MD trajectories, etc.