Another possible explanation is that the synthesis of the porous material was unsuccessful, and the authors reported the nitrogen isotherm for the nonporous sample to document a The second category includes the ratios between 0.1 and 0.75, for which the measured pore volume is substantially lower than the geometric pore volume. Some of the hypotheses mentioned above still apply: partial desolvation, presence of counterions, presence of unreacted ligands trapped in the pores, or partially collapsing (flexible) structures upon activation. We note that that the presence of a guest molecule or a partial collapse of the structure not only reduces the available space for the nitrogen/argon probe molecule but also increases the apparent density of the material: it affects both the numerator and the denominator in the measurement of the pore volume.