Most organic molecules pack in such a way to 1 6 minimize free space, therefore exhibit minimal void volume have previously demonstrated the synthesis of porous organic cages that are permanently porous to a variety of gases. However, study of the static structure alone does not adequately explain the porosity of these materials. This is especially evident in CC3, which takes up a large amount of nitrogen experimentally but its porosity is not obvious from consideration of the computed geometric solvent accessible surface area of the static crystal structure obtained from single crystal X-ray diffraction data. In this study, we show that the structure and flexibility of these organic cages is not well represented by "off the shelf" force fields that have been developed in other areas. Hence, we develop and test a bespoke force field (CSFF) for simulating the molecular dynamics of a series of porous organic cage materials. The development of CSFF has unlocked the ability to investigate phenomena that are difficult to study by direct experiments, for example, molecular dynamic analysis of the window diameters in CC3 has helped to rationalize its high N-2 uptake. In the future, there is much scope to use CSFF to understand the uptake of gases and also larger guests such as halogens and solvents within a whole host of different cage systems leading on to the use of MD analysis for in silico screening of cage materials for particular molecular separations. If reliable, this could be faster than the associated sorption experiments.