Figure shows the quasi-harmonic Helmholtz free energies including zero-point energies for each polymorph calculated as a function of pressure relative to the free energy of Form I. The Figure shows that Forms III and IV become more stable than Forms I and II as the pressure increases above 6 GPa. Experimentally these polymorphs are indeed observed at higher pressures as can be seen in table , though the experimental pressures at 2.6 GPa and 1.7 GPa for polymorphs III and IV respectively are considerable less than the 6 GPa found by calculation.