Widom insertion is a standard calculation method that consists in randomly inserting a single molecule inside an existing molecular system by randomly choosing its center and its rotation angle. By measuring the interaction energy of the molecule inserted, one can obtain the excess free energy ∆𝐹 exc difference associated with its insertion into the framework, i.e., the species chemical potential 𝜇 𝑖 . In the context of adsorption, this method has been used to randomly insert a molecule in the empty porous framework: after many cycles, the simulation has generated a diverse enough sample of points with different interaction energies ℰ int , yielding the adsorption free energy ∆𝐹 ads = -𝑅𝑇 ln(⟨exp(-ℰ int /𝑅𝑇 )⟩), the Henry constant 𝐾 H , as well as the adsorption enthalpy, ∆𝐻 ads (equation 1), which is the opposite sign of the zero-loading isosteric heat of adsorption 𝑄 0 st . The Henry constant 𝐾 H (in mol kg -1 Pa -1 ) associated to the adsorption inside a crystalline framework of mass density 𝜌 𝑓 at temperature 𝑇 can be derived using the following equation 2: