Each experimental polymorph crystal structure was optimised using VASP at a pressure of 0.0 GPa and at the experimental pressure used to determine the structure. The results for the zero pressure optimisations are reported in Tables SI12, SI13. The calculated unit-cell dimensions at the experimental pressure conditions are shown in Table . The percentage deviations from experiment are reported in Table . All deviations are less then 1.6% of the experimental value and there seems to be a general tendency to calculate a smaller unit-cell volume than that which is observed experimentally which is possibly due to temperature effects and highlighted by Form I where the difference between the volume of the calculated cell and that recorded experimentally at 30 K is only 0.91%. The lattice energies quoted in Table are not strictly comparable as some of the calculations are performed under different pressure conditions. However the calculations at zero pressure reported in the SI (Tables &) are all performed at the same pressure and the ranking of the stability of the polymorphs is found to be Form I > Form II > Form IV > Form III.