Thermal expansivities of cubic ice I and ice VII

The free energies of low pressure cubic ice I and high pressure ice VII are calculated over a wide range of temperature in order to examine whether the negative thermal expansivity observed in ice I is a common feature across various ice morphologies. The Gibbs free energies for several proton disordered structures are minimized with respect to volume variation. This enables us to evaluate the thermal expansivity at fixed temperature and pressure from only intermolecular interaction potential. While the negative thermal expansivity of the low pressure ice I in low temperature regime is successfully reproduced, it is found that the expansivity is always positive for the high pressure ice VII.