The idea to exploit different affinities of gases for a surface to purify a gaseous mixture is more than one hundred years old. Since, advanced classes of materials harboring nano-sized pores such as porous cage solids have emerged and offer high adsorptive selectivities. Because the size and shape of the cavity intrinsic to a porous organic cage molecule can strongly influence selectivity for its deployment in a molecular separation or sensing application, it is important to mathematically characterize the void spaces of porous organic cage molecules for predicting adsorption and comparing materials.