  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 Ih, 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 NpH
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.
