Characterizing the stepwise transformation from a low-density to a   very-high-density form of supercooled liquid water
  We explore the phase diagram of TIP4P-Ew [J. Chem. Phys. {\bf 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. 
Dietmar Paschek and Alfons Geiger