To investigate this further, in Figure we zoom into the structures with a measured pore volume below 0.1 cm 3 g -1 and compare them to the pore-limiting diameter computed for the structure. The pore-limiting diameter is the diameter of the largest molecule that can diffuse through the structure, and Figure (b) plots the experimental uptake as a function of the pore-limiting diameter. For structures with a pore-limiting diameter below the kinetic diameter of nitrogen or argon, zero uptake is expected, and these MOFs can be reasonably considered as nonporous. When the pore-limiting diameter is close to the size of the adsorbate, kinetics of diffusion can still be very slow, leading to negligible uptake at low temperature. However, we notice a significant fraction of structures with pore-limiting diameter much larger than the kinetic diameter of nitrogen. In these cases, the measurement may have been conducted on a material that collapsed after solvent removal, solvent removal may have been unsuccessful, or the presence of floating counter-ions may not have been reported in the crystal structure.