Causality in liquid water as a hallmark of emergent glassy dynamics
- 1Condensed Matter and Statistical Physics, The Abdus Salam International Centre for Theoretical Physics, Trieste 34151, Italy
- 2Scuola Internazionale Superiore di Studi Avanzati, Trieste 34136, Italy
Abstract
In molecular liquids such as water, time-delayed influences between microscopic or mesoscopic variables are typically probed using time-correlation functions, which are symmetric under detailed balance and therefore blind to dynamical asymmetries. Here, we characterize water’s dynamics using a causal inference metric that captures asymmetric couplings between collective variables. Analyzing equilibrium molecular dynamics simulations at ambient conditions and in the high-density liquid (HDL) regime of supercooled water, we uncover pronounced asymmetries in the couplings between orientational and translational degrees of freedom across multiple time and length scales. At room temperature, rotational modes exert a modest directional influence on translational dynamics. In contrast, in the supercooled HDL regime, translational motions emerge as the primary drivers of the dynamics, suggesting facilitation-like relaxation mechanisms characteristic of glassy systems. These results reveal a qualitative reorganization of dynamical couplings across thermodynamic conditions, implying that molecular liquids at thermal equilibrium can exhibit an emergent directionality in their fluctuation couplings. As a consequence, our analysis reveals that external perturbations acting on specific degrees of freedom can induce a stronger arrow of time in the causal relations between translational and orientational modes.
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BibTeX
@article{huet2026causality,
title = {{Causality in liquid water as a hallmark of emergent glassy dynamics}},
author = {Léon Huet and Vittorio Del Tatto and Debarshi Banerjee and Alessandro Laio and Ali A. Hassanali},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
year = {2026},
month = oct,
doi = {10.1073/pnas.2616852123},
url = {https://doi.org/10.1073/pnas.2616852123},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - JOUR
TI - Causality in liquid water as a hallmark of emergent glassy dynamics
AU - Léon Huet
AU - Vittorio Del Tatto
AU - Debarshi Banerjee
AU - Alessandro Laio
AU - Ali A. Hassanali
PY - 2026
DA - 2026/10/20
JO - Proceedings of the National Academy of Sciences of the United States of America
DO - 10.1073/pnas.2616852123
UR - https://doi.org/10.1073/pnas.2616852123
AB - In molecular liquids such as water, time-delayed influences between microscopic or mesoscopic variables are typically probed using time-correlation functions, which are symmetric under detailed balance and therefore blind to dynamical asymmetries. Here, we characterize water’s dynamics using a causal inference metric that captures asymmetric couplings between collective variables. Analyzing equilibrium molecular dynamics simulations at ambient conditions and in the high-density liquid (HDL) regime of supercooled water, we uncover pronounced asymmetries in the couplings between orientational and translational degrees of freedom across multiple time and length scales. At room temperature, rotational modes exert a modest directional influence on translational dynamics. In contrast, in the supercooled HDL regime, translational motions emerge as the primary drivers of the dynamics, suggesting facilitation-like relaxation mechanisms characteristic of glassy systems. These results reveal a qualitative reorganization of dynamical couplings across thermodynamic conditions, implying that molecular liquids at thermal equilibrium can exhibit an emergent directionality in their fluctuation couplings. As a consequence, our analysis reveals that external perturbations acting on specific degrees of freedom can induce a stronger arrow of time in the causal relations between translational and orientational modes.
SN - 0027-8424
SN - 1091-6490
ER -