There is also a relationship between the porosity of a material and the bulk moduli, as reported previously in ceramics and zeolite materials. Often these highly porous materials have extremely small bulk modulus, 4.97 and 2.07 GPa for DUT-60 and MOF-399, respectively, which are very low for crystalline solids. Thus, the unique softness of mesoporous MOFs may also be responsible for the remarkable contraction observed.