Pressure-Induced Densification of Ice Ih under Triaxial Mechanical Compression: Dissociation versus Retention of Crystallinity for Intermediate States in Atomistic and Coarse-Grained Water Models.

Molecular-dynamics (MD) simulation of triaxially pressurized ice Ih up to 30 kbar at 240 K (with sudden mechanical pressurization from its ambient-pressure structure) has been carried out with both the single-particle mW and atomistic TIP4P-Ice water potentials on systems of up to ∼1 million molecules, for times of the order of 100 ns. It was found that the TIP4P-Ice systems adopted a high-density liquid state above ∼7 kbar, while densification of the mW systems retained essentially crystalline order, owing to a failure for the tetrahedral network to break down appreciably from its ice Ih lattice structure. Both are intermediate states adopted along the path toward respective thermodynamically stable states (and with pressure removal show reversion to Ih for mW and to supercooled liquid for TIP4P-Ice), similar to recent ice electro-freezing simulations in "No Man's Land". Densification kinetics showed faster mW-system adaptation.