Pressure-Induced Densification of Ice I 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 I 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 I lattice structure. Both are intermediate states adopted along the path toward respective thermodynamically stable states (and with pressure removal show reversion to I 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.