Translational-Rotational Decoupling Dynamics of High-Pressure Liquid Water under Quasi-Isentropic Compression

The ubiquitous presence of water, from Earth and planetary bodies to interstellar space, renders its phase behavior across an extensive thermodynamic range fundamental to understanding key scientific phenomena such as biochemical reactions, climate dynamics, and planetary evolution. Nevertheless, although liquid water exhibits distinct anomalous behaviors under extreme pressure, research has been hampered by experimental limitations and computational complexity, resulting in scarce atomic-scale data and hindered understanding of its microscopic mechanisms. To address this, our study employed a deep learning interaction model trained on high-precision ab initio data. Employing molecular dynamics simulations, we compressed liquid water isentropically to tens of thousands of atmospheres. Systematic analysis of its structural and dynamic properties revealed that elevated pressure significantly disrupts the inherent tetrahedral local coordination of water molecules, enhancing their rotational mobility. Conversely, translational mobility is severely suppressed in this highly condensed state. The mean squared displacement of water molecules under high pressure exhibits a characteristic three-stage behavior which is typical of glassy systems: ballistic transport, a plateau, and diffusion. Macroscopically, this reduced translational mobility manifests as a substantial increase in shear viscosity. A critical finding is that, unlike supercooled water under ambient pressure where translational and rotational motions are strongly coupled, liquid water under dynamic high pressure exhibits an intrinsic decoupling of these motions. The insights from this work are expected to offer significant microscopic understanding for crucial scientific questions, including the response of materials under dynamic loading and the solidification of metastable liquids.