The crystal structures are relaxed allowing variation of the unit cell lengths and angles, and atom positions until all forces are less than 0.01 eV Å-1 . After the initial convergence of each DFT-D optimisation, calculations are restarted, sometimes multiple times, to mitigate the effects of Pulay stresses until the optimal geometry and energy converge. To obtain the reference energy values, it is necessary to also evaluate the monomer energies for all molecules in the asymmetric unit cell. From the relaxed crystal structure, each distinct molecule, m, in the asymmetric unit cell is placed in a fixed cubic unit cell with a minimum inter-molecule contact distance of at least 16 Å. It is assumed that, at this separation, intermolecular interactions are negligible. A single-point calculation is conducted to obtain the monomer energy at the crystalline conformation (U mono,crys,m ) while an optimisation is conducted to obtain the corresponding energy of the monomer in vacuo (U mono,vac,m ). The desired energy values for a crystal structure can then be obtained as,