where U vasp is the optimal crystal energy reported by VASP, while U inter,ref , ∆U intra,ref , and U latt,ref are the intermolecular, intramolecular and lattice energy of the crystal derived from the VASP calculations. Z and Z ′ correspond to the number of molecules in the unit cell and asymmetric unit cell respectively. From the output of VASP, the symmetry relations for each of the optimised structures are recalculated using PLATON . In some cases, the symmetry of the crystal cannot be reduced to that of the experimental structure, in which case the system is treated as Z ′ > 1.