The lattice constants of the five filled models, viz. HeCo8Sb24, NeCo8Sb24, ArCo8Sb24, KrCo8Sb24 and XeCo8Sb24, are listed in Table . Filling with Ne-Xe results in a small expansion of the unit cell, increasing in proportion with the atomic radii of the fillers, and the maximum 0.64 % expansion with Xe suggests the cavity is large enough to accommodate all five atoms. Geometry optimisations including the DFT-D3 dispersion correction produced lattice constants around 0.75 % smaller with the same overall trend, suggesting that a dispersion correction is not required for these systems (Table .1).