For each configuration, the dimers were geometry optimised using OPLS4, constraining the atomic positions of the vertices of the cage to maintain the relative position/angles of the two cages. Dimers that were within 50 kJ mol -1 of the lowest energy configuration for their packing type were then geometry optimised with no constraints. Unconstrained configurations that were within 30 kJ mol -1 of the lowest energy configuration were then further geometry optimised using DFT with the mixed Gaussian and plane wave code CP2K/QUICKSTEP 23 with the PBE functional, GTH-type pseudopotentials, molecular optimised TZVP-MOLOPT basis sets 26 for all atoms and the Grimme-D3 dispersion correction. Details of the convergence criteria are given in the Section S1. From the constrained dimer calculations, the window-to-window dimers had the lowest energy structures, followed by the window-to-arene, and then the arene-to-arene (Table ).