Structural and dynamic anomalous properties of TIP4P/2005 water at extreme pressures

TL;DR

This study uses extensive molecular dynamics with the TIP4P/2005 water model to explore structural and dynamic anomalies of water under extreme pressures and moderate to low temperatures. By computing self-diffusion, shear and bulk viscosities, structural metrics (RDF, $S(q)$, translational order parameter), and the structural relaxation time $\\tau_{\\alpha}$ across five isotherms (220–300 K) and up to 2.7 GPa, the work reproduces experimental trends and confirms a minimum in $\\tau_{\\alpha}$ with pressure, which shifts with temperature. The microscopic analysis links this anomaly to abrupt reorganization of the hydrogen-bond network and concomitant changes in the second coordination shell, providing a cohesive picture of how pressure drives coupled structural and dynamical responses in water. The results underscore a nested pattern of anomalies and offer mechanistic insight into water’s non-Arrhenius behavior, with implications for experiments probing water under high pressure and low temperature.

Abstract

Water shows numerous thermodynamic, dynamic, and structural anomalies. Recent experiments [Eichler et al. Phys. Rev. Lett. 134, 134101 (2025)], based on measurements of shear and bulk viscosities of liquid water up to 1.6 GPa, have reported the existence of a minimum in the variation of the structural relaxation time τα with pressure at room temperature. Here we investigate this and related properties with molecular dynamics simulations of the TIP4P/2005 water model, performed at extreme pressures commensurate with the experiments. Specifically, we compute dynamic (self-diffusion, shear and bulk viscosities, and structural relaxation time) and structural (oxygen-oxygen radial distribution function and structure factor, translational order parameter) properties down to 220 K and up to 2.7 GPa. We find good agreement with the experimental observations, and confirm the existence of a minimum in τα . The microscopic information obtained from the simulations suggests that this anomaly is connected with the sudden reorganization of the hydrogen bond network induced by pressurization.

Figures (11)

• Figure 1: Equation of state calculated for T = 220 K (a, purple diamonds), 240 K (b, green inverted triangles), 260 K (c, blue squares), 280 K (d, red circles), and 300 K (e, black triangles).
• Figure 2: Shear $\eta$ (a) and bulk $\kappa$ (b) viscosities as a function of pressure $P$ computed for $T =$ 260 K (blue squares), 280 K (red circles), and 300 K (black triangles). Filled symbols correspond to the present work, and empty symbols to previous simulations from Ref. montero2018viscosity (a) and Ref. fanourgakis2012determining (b). The solid red curves display polynomial fits to the experimental results at 295 K eichler2025shear(note that the experimental minima in $\eta$ and $\kappa$ are not well captured by the fits on this scale).
• Figure 3: Ratio between bulk ($\kappa$) and shear viscosity ($\eta$) as a function of pressure $P$ for 260 K (blue squares) , 280 K (red circles) and 300 K (black triangles). The solid red curve displays the experimental results at 295 K eichler2025shear.
• Figure 4: Self-diffusion coefficient $D$ as a function of pressure $P$ for 220 K (purple diamonds), 240 K (green inverted triangles), 260 K (blue squares) , 280 K (red circles) and 300 K (black triangles). Filled symbols correspond to the present work, and empty symbols to previous simulations from Ref. montero2018viscosity
• Figure 5: (a-e) Radial distribution function $g(r)$; (f) distance for the first minima in radial distribution function, $r_\mathrm{min}$ (inverted triangles), and distances for the two first maximum values, $r_1$ (diamonds) and $r_2$ (circles); (g) coordination number computed for the first minima in radial distribution function, $n_\mathrm{min}=n(r_\mathrm{min})$ (inverted triangles), and for some constant distance $n_{3.0}=n(r_c=3.0$Å$)$ (diamonds) and $n_{3.5}=n(r_c=3.5$Å$)$ (circles); and (h) translational order parameter $t$, calculated for all densities/pressures at temperatures $T =$220 K (purple), 240 K (green), 260 K (blue), 280 K (red), and 300 K (black).