Thermodynamic and Structural Characterization of the Transformation from a Metastable Low‐Density to a Very High‐Density Form of Supercooled TIP4P‐Ew Model Water

We explore the phase diagram of the metastable TIP4P‐Ew liquid model water from 360 K down to 150 K at densities ranging from 0.950 to 1.355 g cm^?3. In addition to the low‐density/high‐density (LDL/HDL) liquid–liquid transition, we observe a structural high‐density/very high‐density (HDL/VHDL) transformation for the lowest temperatures at 1.30 g cm^?3. The characteristics of the isobars and isotherms suggest the presence of a stepwise HDL/VHDL transition with first‐order‐like appearance. In addition, we also identify an apparent pretransition at 1.24 g cm^?3, which suggests that the experimentally detected LDA/VHDA transformation might evolve into a multiple‐step process with different local structures representing local minima in the free‐energy landscape. Such a scenario is supported by a pronounced correlation between the isothermal density dependence of the pressure, with a stepwise increase of the oxygen coordination number, due to the appearance of interstitial water molecules.