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

• Received 18 November 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.193003