Communication: Long-range angular correlations in liquid water.

At ambient conditions the intermolecular correlation in liquid water is generally believed to be short ranged as shown in the atomic pair distribution functions (PDFs) obtained from scattering experiments or from theoretical predictions. However, atom-atom PDFs provide only a partial description of the higher dimensional intermolecular correlation function that depends on both the positions and orientations of water molecules. Here we study the atomic PDFs of liquid water as well as the angular correlation function (ACF) using a classical density functional theory. We demonstrate that, different from the PDFs, the ACF exhibits long-range oscillatory decay extending up to tens of molecular diameters. The theoretical predictions are in good agreement with molecular simulations and corroborate recent experimental results from the second harmonic light scattering experiments.