Another approach to obtain mechanical properties at a given temperature T is to evaluate them from the fluctuations of the unit cell of an equilibrated system simulated under a constant-stress (N, σ, T ) ensemble. Unlike the previous finite difference methods which This approach, already used for the ZIF-4 crystal with a classical force field, requires long equilibration times (∼ 5 -10 ns). While out of reach of AIMD simulations, it is tractable with ReaxFF. The bulk modulus can also be directly computed from the fluctuations of the volume, a second method we used as a validation.