for the glass and 1.50 g cm -3 for the crystal 41 indicate that a larger value is expected for the glass. Although we could expect the difference in crystallographic densities between the glass and crystal to be much smaller, the complex diffusion behavior of helium into the ZIF-4 pores makes pycnometric densities measurements difficult to interpret. As an alternative to pycnometry, a recent work determined the crystallographic density for the ZIF-4 glass from CO 2 physisorption studies. Estimated to be 1.38 g cm -3 , this value hints that ReaxFF overestimates the glass density.