In order to evaluate the robustness of such constant-pressure simulations, we equilibrated at T = 300 K and P = 0 Pa several ZIF-4 systems: the crystal, three ab initio glasses models from Ref. 10 and a glass obtained by Reverse Monte Carlo (RMC) modeling. These systems, thoroughly presented in the supporting information, went through the preparation process detailed in the first section . They were then equilibrated between 5 ns and 20 ns in the (N, P, T ) ensemble with a flexible cell. More general than an isotropic cell, a flexible cell enables the computation of multiple mechanical properties using the strain-fluctuation method, 47 already applied to ZIFs using classical force fields. 11 As reported on Table , we see that the density of every equilibrated system is in the range of 1.5 to 1.7 g cm -3 , regardless of their initial density. We conclude that the large density observed for the ReaxFF glass may not be due to its specific structure, but is rather a symptom of ReaxFF tendency to densify systems. We have found that using an isotropic cell for the crystal and ab initio glasses reduces the magnitude of the densification respectively by 0.25 g cm -3 (see Figure ) and 0.14 g cm -3