The empty-host free energy was obtained by thermodynamic integration of the internal pressure, extracted from MD simulations of the empty framework in the (N, V, σa = 0, T ) ensemble at different unit cell volumes at 120 K. As the entropic contribution of the empty framework is negligible, the obtained free energy profile was also used to construct the osmotic potential at 90 K and 150 K. The free energy was chosen to be 0 kJ mol -1 in the op phase at a unit cell volume of 104 nm 3 ,which is comparable to the experimentally observed unit cell volume of 100 nm 3 , 4 for a meaningful comparison of the empty-host free energy (and thus the osmotic potential). To obtain accurate integrals of the number of adsorbed particles (represented by the third term in Equation ), simulations were performed between 46 and 75 values of gas pressure for each of the three considered temperatures. The chemical potential at each temperature and gas pressure was calculated with the Peng-Robinson equation of state. The above approach produces the osmotic potential at different methane gas pressures and cell volumes, as displayed in Figure (at 120 K). In contrast to previous approaches (Figure ), this surface offers a tremendous amount of additional information. The osmotic surface illustrates both metastable and equilibrium states and their evolution with increasing gas pressure. At low to intermediate gas pressures, there exists a clear osmotic barrier between the open pore (op) and contracted pore (cp) phases, leading to preservation of the metastable op phase.