We classify a given point in/around a cage as void space, as opposed to overlapping an atom of the cage, by computing the potential energy of a helium adsorbate at that point to determine if its interaction with the cage is dominantly repulsive. If and only if the potential energy is less than k B T , with k B the Boltzmann constant and T = 298 K the temperature, the point is classified as void space. We model the energetics of the interaction of a helium atom with the atoms of the cage as pairwise additive and with 12-6 Lennard-Jones potentials, taking parameters from the Universal Force Field 48 (geometric mixing rules, cutoff radius 14 Å). We use PorousMaterials.jl v0.1.3 49 to compute the potential energies.