Advanced porous materials can deliver exciting developments to industrial processes. In particular, the flexibility of porous adsorbents enables exceptional performance in the storage and separation of gases. Responsive adsorbents, such as metal-organic frameworks (MOFs), can significantly change pore structure and volume in response to external stimuli such as pressure, temperature, gas adsorption or light absorption. One particularly exciting MOF that exhibits negative gas adsorption (NGA) is DUT-49. First reported in 2016, this MOF undergoes a structural transition associated with a 50% contraction of the unit cell upon increasing methane gas pressure, resulting in spontaneous desorption of methane. This release of methane leads to pressure amplification in a closed system, opening the door to programmable pneumatics systems. The lack of thermodynamic understanding of the NGA phenomenon has motivated a number of computational studies. However, efforts have mostly relied on two-state models that are unable to capture the complete phase-space during adsorption and desorption. The most appropriate description of the thermodynamics of responsive frameworks under gas adsorption is provided by the osmotic ensemble. Simulations in this ensemble im- pose a chemical potential of the adsorbed fluid (µ), a mechanical pressure (p) and a temperature (T ). Under these conditions, the equilibrium framework minimizes the osmotic potential (Ω) through structural relaxation, changes in cell shape and volume, and adsorption of guest molecules. 9 However, atomistic simulations within the osmotic ensemble remain challenging because dynamics must be combined with particle insertion/deletion, limiting its application to a handful of systems. Descriptions of the osmotic potential are often achieved through analytical treatment of gas adsorption. This has been particularly insightful for the relative stability of two distinct and porous phases. For example, the process of "breathing" between an open (op) and closed porous phase (cp) can be understood by their difference in osmotic potential, ∆Ω = Ωop -Ωcp (Figure ). The difference in osmotic potential reveals which phase represents the global thermodynamic equilibrium as a function of gas pressure. (Note that the gas pressure equals the mechanical pressure and also affects the chemical potential of the fluid). However, this methodology is not sufficient to describe NGA. Krause et al. showed that the structural transition associated with NGA must involve metastable states, as it cannot occur in global thermodynamic equilibrium. Moreover, the experimentally observed hysteresis and temperature dependence of NGA cannot be reproduced by sim-ply considering the difference in osmotic potential. Therefore, knowledge of not only the osmotic potential of both phases, but also the osmotic barriers that exist between both phases that act to prevent phase transformations of both collapse and reopening, keeping the system in a metastable state, are required.