Long-Range Angular Correlations in Liquid Water

The microscopic structure of liquid water is of fundamental importance to our understanding of aqueous systems including hydrophobic phenomena and vast biological processes at the microscopic scales. In contrast to that for simple fluids, the local structure of liquid water reflects not only the molecular excluded volume effects but also a partially tetrahedral network of hydrogen bonds among water molecules.  However, it is commonly believed that, near ambient conditions, the structure of liquid water lacks long-range correlations as shown in the structure factor or the pair distribution functions obtained from by x-ray or neutron diffraction techniques or from molecular simulations.  In this work, we demonstrated, by using a molecular density functional theory, the existence of long-range orientational correlations in liquid water at a length scale of about 40 Å. The angular correlation function attenuates in an oscillatory manner consistent with recent experimental studies. We also examined the angle dependence of density profile and found that the long-range correlations are mainly in the dipolar directions and little influenced by the rotational angles.