Anomalous nuclear quantum effects in ice

One striking anomaly of water ice has been largely neglected and never explained. Replacing hydrogen (1H) by deuterium (2H) causes ice to expand, whereas the normal isotope effect is volume contraction with increased mass. Furthermore, the anomaly increases with temperature T, even though a normal isotope shift should decrease with T and vanish when T is high enough to use classical nuclear motions. In this study, we show that these effects are very well described by ab initio density-functional theory. Our theoretical modeling explains these anomalies, and allows us to predict and to experimentally confirm a counter effect, namely, that replacement of 16O by 18O causes a normal lattice contraction.