The quasi-harmonic approximation may not be appropriate in this system as the mobility of the hydrogen bond network in Form I indicates that large amplitude anharmonic motion may be present at room temperatures. Another possible contribution to the experimental polymorph stability not considered in the computational work is the contribution from the disorder of the water content in these hygroscopic crystals. As well as giving rise to additional electronic and vibrational contributions to the free-energy, the disorder will also give rise to a configurational entropy contribution which we expect to be larger for lower pressures and smaller for smaller unit-cells. The effect of this would be to make this term important in determining the crossing points in relative stability of the polymorphs as a function of pressure.