  When two phases of water are at equilibrium, the ratio of hydrogen isotopes
in each is slightly altered due to their different phase affinities. This
isotopic fractionation process can be utilized to analyze water's movement in
the world's climate. Here we show that equilibrium fractionation ratios, an
entirely quantum mechanical property, also provide a sensitive probe to assess
the magnitude of nuclear quantum fluctuations in water. By comparing the
predictions of a series of water models, we show that those describing the OH
chemical bond as rigid or harmonic greatly over-predict the magnitude of
isotope fractionation. Models that account for anharmonicity in this coordinate
are shown to provide much more accurate results due to their ability to give
partial cancellation between inter and intra-molecular quantum effects. These
results give evidence of the existence of competing quantum effects in water
and allow us to identify how this cancellation varies across a wide range of
temperatures. In addition, this work demonstrates that simulation can provide
accurate predictions and insights into hydrogen fractionation.
