A complete theory should also be able to predict the reverse transition upon heating. In a second-order phase transition, this should manifest itself in a similar phonon mode softening upon increasing temperature, and a convergence of the free energies of the two phases at the phase transition temperature. However, no softening of the phonon modes is observed for the tetragonal polymorph in Fig. . The tetragonal phase is also predicted to be 20-40 meV atom 1 more stable at all temperatures when comparing both harmonic and anharmonic free energies (Fig. , S6 and Fig. ).