Order–disorder phase boundary between ice VII and VIII obtained by first principles

We report fundamental progress in the description of an archetypical hydrogen-bonded system: H 2 O –ice. We calculated by first principles the ice VII–VIII phase boundary, an order–disorder phase boundary, by performing a complete sampling of configurations in a 16-molecule supercell. Vibrational effects are included within quasiharmonic theory. Several important features were successfully described: negative Clapeyron slope, isotope effect, and nearly-constant T c at low pressures. The equation of state was very well described below 40 GPa. Inclusion of tunneling effects and increase of supercell size should decrease the remaining discrepancies, i.e., the overestimation of volume above ∼40 GPa and the underestimation of T c .