We investigate the structural properties of liquid water at near ambient
conditions using first-principles molecular dynamics simulations based
on a semilocal density functional augmented with nonlocal van der Waals
interactions. The adopted scheme offers the advantage of simulating
liquid water at essentially the same computational cost of standard
semilocal functionals. Applied to the water dimer and to ice I-h, we
find that the hydrogen-bond energy is only slightly enhanced compared to
a standard semilocal functional. We simulate liquid water through
molecular dynamics in the N pH statistical ensemble allowing for
fluctuations of the system density. The structure of the liquid departs
from that found with a semilocal functional leading to more compact
structural arrangements. This indicates that the directionality of the
hydrogen-bond interaction has a diminished role as compared to the
overall attractions, as expected when dispersion interactions are
accounted for. This is substantiated through a detailed analysis
comprising the study of the partial radial distribution functions,
various local order indices, the hydrogen-bond network, and the
selfdiffusion coefficient. The explicit treatment of the van der Waals
interactions leads to an overall improved description of liquid water.
