This trend was also seen for the unconstrained dimer calculations. The only dimers that were within 30 kJ mol -1 of the lowest energy configuration for the unconstrained dimer calculations, and thus taken forward for DFT calculations, had a window-to-window packing arrangement (Table ). The DFT calculations showed that the lowest energy configuration of B11 is when the windows of the cages are anti-aligned, but slightly slipped off the centre of the facets (Fig ). In this configuration, the π -π interactions are maximised as the arenes on the vertices of the cages rotate to form π -π stacking with each other, as well as with the arene on the truncated facet of the tetrahedra. This leads to 4 π -π bonds between the two cages.