Nuclear Quantum Effects in Liquid Water Are Negligible for Structure but Significant for Dynamics

TL;DR

This study uses advanced simulations to show that nuclear quantum effects significantly influence the dynamics of liquid water but have negligible impact on its structure, clarifying longstanding experimental controversies.

Contribution

It provides the first converged coupled cluster-quality path integral simulations comparing H₂O and D₂O, revealing the differential impact of nuclear quantum effects on water's structure and dynamics.

Findings

Quantum effects strongly affect water's rotational and translational dynamics.

Structural differences due to quantum effects are minimal, with fluctuations being more affected.

Water's structure remains nearly unchanged, but fluctuations increase, making H₂O more 'liquid' than D₂O.

Abstract

Isotopic substitution, which can be realized both in experiment and computer simulations, is a direct approach to assess the role of nuclear quantum effects on the structure and dynamics of matter. Yet, the impact of nuclear quantum effects on the structure of liquid water as probed in experiment by comparing normal to heavy water has remained controversial. To settle this issue, we employ a highly accurate machine-learned high-dimensional neural network potential to perform converged coupled cluster-quality path integral simulations of liquid H$_2$O versus D$_2$O at ambient conditions. We find substantial H/D quantum effects on the rotational and translational dynamics of water, in close agreement with the experimental benchmarks. However, in stark contrast to the role for dynamics, H/D quantum effects turn out to be unexpectedly small, on the order of 1/1000 \r{A}, on both intramolecular and H-bonding structure of water. The most probable structure of water remains nearly unaffected by nuclear quantum effects, but effects on fluctuations away from average are appreciable, rendering H$_2$O substantially more "liquid" than D$_2$O.