Nuclear Quantum Effects on Molecular Packing in Light and Heavy Water

Light water shows a temperature of maximum density (TMD) at 4$^{\circ}$C, below which the liquid begins to expand upon cooling. Hydrogen bonding is thought to be responsible for the anomalous density behaviour below the TMD: molecules in the liquid must move apart in order to form hydrogen bonds which acts to drive down the density. The density maximum is thought to mark the point during cooling at which formation of an open hydrogen-bonded tetrahedral network becomes energetically preferred over contraction familiar in simple liquids. In heavy water, the TMD leaps upwards to 11$^{\circ}$C. The lower TMD in light water might be supposed to be a result of breakdown in the open hydrogen-bonded network due to NQD: orientational delocalisation of molecules weakens hydrogen bonding and allows a higher density at a lower temperature. We have analysed first- and second-nearest-neighbour distances in relation to molecular orientation using results from path integral molecular dynamics simulation. A simple breakdown in tetrahedral order upon density increase due to orientational delocalisation is not found in light water. Rather, NQD in light water permits hydrogen bonds to become \emph{straighter} at higher density. The result is to increase the size of cavities in the tetrahedral network and thus provide accommodation for a greater number of interstitials which find homes in those cavities. A qualitative explanation for the phenomena is given in terms of the energetics of the water dimer.