The static free energy calculations showed that Forms III and IV become more stable than Form I or II as pressure increases. This agrees with the experimental stability of these polymorphs only at higher pressures. However, the pressure at which this cross-over was calculated to occur (6 GPa) is considerably greater than the pressures required experimentally (2.6 GPa and 1.7 GPa for Forms III and IV respectively). This may be due to the inclusion of only harmonic contributions to the free energy, when it is clear from the IR spectral calculations presented here that there is a high degree of anharmonicity in these systems, particularly at low frequencies, that will significantly effect the vibrational contribution to the free energies in these systems.