The symmetry-independent (unique) guest molecules were extracted from the DFT optimised experimental crystal structures. Single-point energy calculations were performed to obtain the energies of the isolated molecules from their crystalline state, E mol crystal (see red point in Fig. ). The crystalline molecular geometry was also used to obtain the associated local energy minimum on the conformational energy surface, E opt,mol local , by running unconstrained geometry optimisations to relax the molecule to the nearest local energy minimum (blue point marked LM in Fig. ). The energy difference between the single-point and relaxed molecular geometries of the unique molecules within the crystal structures is the intramolecular strain energy, ∆E strain (see Fig. and Eq. 1):