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The structure of liquid water at ambient conditions is studied in ab initiomolecular dynamics simulations using van der Waals (vdW) density-functionaltheory, i.e. using the new exchange-correlation functionals optPBE-vdW andvdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highlydirectional hydrogen-bonds, which are enhanced by standard functionals. Thisbrings about a softening of the microscopic structure of water, as seen fromthe broadening of angular distribution functions and, in particular, from themuch lower and broader first peak in the oxygen-oxygen pair-correlationfunction (PCF), indicating loss of structure in the outer solvation shells. Incombination with softer non-local correlation terms, as in the newparameterization of vdW-DF, inclusion of vdW interactions is shown to shift thebalance of resulting structures from open tetrahedral to more close-packed. Theresulting O-O PCF shows some resemblance with experiment for high-density water(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but notdirectly with experiment for ambient water. However, an O-O PCF consisting of alinear combination of 70% from vdW-DF2 and 30% from experiment on low-densityliquid water reproduces near-quantitatively the experimental O-O PCF forambient water, indicating consistency with a two-liquid model with fluctuationsbetween high- and low-density regions.

Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like