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Abstract
Mounting experimental evidence supports the existence of a liquid-liquid transition (LLT) in high-pressure supercooled water. However, fast crystallization of supercooled water has impeded identification of the LLT line TLL(p) in experiments. While the most accurate all-atom (AA) water models display a LLT, their computational cost limits investigations of its interplay with ice formation. Coarse-grained (CG) models provide over 100-fold computational efficiency gain over AA models, enabling the study of water crystallization, but have not yet shown to have a LLT. Here we demonstrate that the CG machine-learned water model ML-BOP has a LLT that ends in a critical point at pc = 170±10 MPa and Tc = 181±3 K. The TLL(p) of ML-BOP is almost identical to the one of TIP4P/2005, adding to the similarity in the equation of state of liquid water in both models. Cooling simulations reveal that ice crystallization is fastest at the liquid-liquid transition and its supercritical continuation of maximum heat capacity, supporting a mechanistic relationship between the structural transformation of water to a low-density liquid and ice formation. We find no signature of liquid-liquid criticality in the ice crystallization temperatures. ML-BOP repli-cates the competition between formation of low-density liquid (LDL) and ice observed in ultrafast experiments of decompres-sion of the high-density liquid (HDL) into the region of stability of LDL. The simulations reveal that crystallization occurs prior to the coarsening of the HDL and LDL domains, obscuring the distinction between the highly metastable first order LLT and pronounced structural fluctuations along its supercritical continuation.
Supplementary materials
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Supporting Information
Description
Presents supporting methodology and supporting results
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Title
Movie 1 - Simulation trajectory of decompression of HDL to produce LDL and ice at 215 MPa
Description
Evolution of a simulation cell with 216000 water molecules along an isobaric isoenthalpic (NpH) simulation that instantaneously decompresses HDL from 160 K and 400 MPa to 215 MPa. Ice is shown with cyan balls and sticks, 4-coordinated molecules L with blue balls, and higher-coordination molecules with red balls. The movie shows the evolution over the first 21 ns, with configurations every 10 ps.
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Movie 2 - Evolution of S(q) as the HDL decompresses to form LDL and ice at 215 MPa
Description
Evolution of the structure factor S(q) for a system of 216000 water molecules along an isobaric isoenthalpic (NpH)decompression simulation from 160 K and 400 MPa to 215 MPa. The dotted lines in pink and green indicate the reference peaks in S(q) for HDL and LDL, respectively. The pink region denotes the S(q) corresponding to our initial HDL configuration of the simulation and blue region presents its evolution along the simulation.
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Movie 3 - Simulation trajectory of decompression of HDL to produce LDL and ice at 170 MPa
Description
Evolution of a simulation cell with 216000 water molecules along an isobaric isoenthalpic (NpH) simulation that instantaneously decompresses HDL from 160 K and 255 MPa to 170 MPa. Ice is shown with cyan balls and sticks, 4-coordinated molecules L with blue balls, and higher-coordination molecules with red balls. The movie shows the evolution over the first 21 ns, with configurations every 10 ps.
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Movie 4 - Simulation trajectory (showing only ice) along the decompression of HDL to produce LDL and ice at 170 MPa
Description
Formation of ice (cyan) in a system of 216000 water molecules along isoenthalpic decompression simulations from 255 MPa, 160 K to 170 MPa for 21 ns. Each frame of the movie represents 10 ps of the simulation trajectory. Ice comprises cubic and hexagonal ice identified with CHILL+.
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