Anomalous Nuclear Quantum Effects in Ice

One striking anomaly of water ice has been largely neglected and never explained. Replacing hydrogen (H-1) by deuterium (H-2) 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 O-16 by O-18 causes a normal lattice contraction.