To estimate the energy barrier of solvent activation in MOFs, we used TMMC to calculate the relative 150 free energy of solvents adsorbed within a MOF as a function of solvent density at a constant fugacity By analysing the magnitude of ΔU ‡ act at coexistence for different MOF-solvent pairs, it is possible to 160 investigate the mechanism and energetics of solvent activation. As an example, we present data for dichloromethane and acetonitrile in IRMOF-1 in Figure which are known to lead to successful activation and activation-collapse, respectively. For both sorbates, phase coexistence is observed between a high density phase at ca. 800 kg/m 3 and a low density phase at <50 kg/m 3 (at ca. 4400 Pa and 490 Pa for dichloromethane and acetonitrile, respectively). The free energy of the phase transition 165 was very distinct, however -ΔU ‡ act was significantly lower for dichloromethane (ca. 1.1 kJ/mol(Zn)) than for acetonitrile (ca. 3.1 kJ/mol(Zn)), suggesting that the key difference between behaviour of the two solvents lies in the nature of the phase change.