Characterizing the stepwise transformation from a low-density to a very-high-density form of supercooled liquid water

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We explore the phase diagram of TIP4P-Ew [J. Chem. Phys. 120, 9665 (2004)] liquid model water from the boiling-point down to 150 K at densities ranging from 0.950 g cm⁻³ to 1.355 g cm⁻³. In addition to the low-density/high-density (LDL/HDL) liquid-liquid transition, we observe a high-density/very-high-density (HDL/VHDL) transformation for the lowest temperatures at 1.30 g cm⁻³. A Van der-Waals type loop suggest the presence of a first order HDL/VHDL transition. In addition, we identify a pre-transition at 1.24 g cm⁻³, suggesting the experimentally detected HDA/VHDA-transformation to be a two-step process. For both pre- and main- HDA/VHDA-transition we observe a step-wise increase of the oxygen coordination number for interstitial water molecules.