Scrutinizing oxygen isotope effects in liquid water with coupled cluster accuracy

Nuclear quantum effects significantly affect the properties of liquid water, in particular due to the quantum delocalization of protons, and differences between normal water (H2O) and heavy water (D2O) are detectable in experiments and simulations. In addition to H/D substitution, oxygen isotope substitution (16O/18O) can be used in experiments to extract partial structure factors and to obtain structural insights. That raises the question of whether the structures of normal water and 18O-substituted water are indeed identical, as such approaches would assume. We perform converged path integral simulations with CCSD(T) accuracy of liquid H2natO and H218O, made possible by intertwining accurate machine-learned high dimensional neural network potential methodology with efficient near-linear scaling coupled cluster theory, and quantify structural differences under ambient conditions. We find statistically significant changes in the water structure upon oxygen isotope substitution-thus being manifestations of nuclear quantum effects. They lead to enhanced radial oxygen-oxygen correlations in the first shell of H218O of up to 0.5% compared to normal water, followed by less pronounced changes that extend even to the second and third shells. Interestingly, we also discover that the quantum delocalization of the hydrogen atoms is approximately 0.1% smaller in H218O than in H2natO due to oxygen isotope effects, that linear hydrogen bond configurations are favored in H218O relative to H2natO, and that hydrogen atoms in H218O are displaced slightly more toward hydrogen bond acceptors.