Molecules which would typically be considered rigid are slightly distorted and therefore have a non-zero total relative energy: we calculate values of ∆E total =2.50 kJ/mol for 4trifluoromethylphenyl azide (CSD Refcode: XAZPAS), and 3.99 kJ/mol for (+)-artemisinin (XAZPEW), which has zero rotatable bonds, excluding terminal methyl groups (Fig. ). Although the geometric difference is small, this demonstrates the crystal packing effects present in CS structures induce distortions of even the most rigid of molecules. The lowest calculated ∆E total is 2.42 kJ/mol for one of the four guest molecules of vanillin (XAZNUK) and one third (9 of 27) of molecules studied have a calculated ∆E total less than 5 kJ/mol (approximately 2RT at room temperature). These mostly correspond to the smaller molecules with few rotatable bonds (Table ): vanillin, (+)-artemisinin, transanethole, 4-trifluoromethylphenyl azide and 1R-(-)-menthyl acetate.