Many attempts at understanding the adsorption-induced flexibility of MOFs initially relied on simple theoretical (or analytical) models featuring qualitative agreement with experimental behavior, in large part because sampling the full thermodynamic and phase space in the osmotic ensemble was deemed too difficult for direct molecular simulation. While such models continue to be refined and have value in providing fundamentally simple explanations of the driving factors for flexibility, they have recently been supplemented with more direct molecular simulation methods. These advanced simulations employ free energy methods (or extended free energy methods) in order to compute thermodynamic properties of interest, including the osmotic potential of the system Ω(N host , µ i , σ, T ), across a vast thermodynamic space. In doing so, they can identify the full behavior of the flexible system, including the occurrence of stable and metastable phases, and determine the structural transitions that take place (or not) upon adsorption. In order to describe both the thermodynamic equilibrium of the adsorbed phase and the internal degrees of freedom of the host framework, such simulations typically require the coupling of Monte Carlo (MC) and molecular dynamics (MD) steps in what is known as a hybrid MC/MD (depicted in Figure ): this ensures proper sampling of the thermodynamic ensemble of choice, the osmotic ensemble.