Nuclear Quantum\nEffects in Liquid Water Are Marginal\nfor Its Average Structure but Significant for Dynamics

Isotopic substitution, which can be realized in both\nexperiment\nand computer simulations, is a direct approach to assess the role\nof nuclear quantum effects on the structure and dynamics of matter.\nHowever, the impact of nuclear quantum effects on the structure of\nliquid water as probed in experiment by comparing normal to heavy\nwater has remained controversial. To settle this issue, we employ\na highly accurate machine-learned high-dimensional neural network\npotential to perform converged coupled cluster-quality path integral\nsimulations of liquid H<sub>2</sub>O versus D<sub>2</sub>O at ambient\nconditions. We find substantial H/D quantum effects on the rotational and translational dynamics of\nwater, in close agreement with the experimental benchmarks. However,\nin stark contrast to the role for dynamics, H/D quantum effects turn\nout to be small, on the order of 1/1000 Å, on both average intramolecular\nand H-bonding structures of water. The most probable structure of\nwater remains nearly unaffected by nuclear quantum effects, but effects\non fluctuations away from average are appreciable, rendering H<sub>2</sub>O substantially more “liquid” than D<sub>2</sub>O.