In Section 3.3 (Table ), it was shown that the total lattice energy of form I is calculated to be marginally lower than that of form II at absolute zero. To further investigate the relative stability of the polymorphs at real temperatures, the Gibbs free energy curves of both forms were calculated from 7K to 300K (shown in Figure and the lower bound being limited by the CRYSTAL17 software package 28 ). A turning point is predicted at 75K, where the Gibbs free energy of form I becomes larger than that of form II, which infers that form II is more stable at ambient temperature. The dynamic calculations show specifically that the stability of form II arises as a result of its greater vibrational entropy. However, two caveats apply: (1) the free energy calculation is based on the harmonic approximation, which would be expected to influence the predicted value of the transition point; (2) the calculations refer to the idealised form I and form II crystal structures, and do not consider structural disorder associated with the intergrown nature of form I / form II single crystals.