Structural transitions in liquid water at high temperatures and pressures: Evidence from molecular simulations

Classical molecular dynamics simulations were carried out for liquid water using the TIP4P-2005 potential model along the 50 MPa isobar, covering a wide temperature range from ambient to near-critical conditions. Particular attention was given to the behavior of various local structural descriptors of liquid water, as well as to the corresponding dynamics and entropic quantities. The results obtained reveal the existence of two distinct structural transitions, located in the temperature range around 423.15 and 498.15 K, respectively. The observed transitions have been characterized by local extrema and crossovers in many of the above-mentioned quantities. Significant changes in the hydrogen bond network were also observed across these transitions. These structural rearrangements are reflected in the calculated intermolecular vibrational and librational dynamics, as evidenced by clear modifications in the spectral densities of atomic velocity correlation functions and in the translational and rotational densities of states.