  We present results for a density functional theory study of small water
clusters and hexagonal ice Ih, using the van der Waals density functional
(vdW-DF). In particular, we examine energetic, structural, and vibrational
properties of these systems. Our results exhibit excellent agreement with both
experiment and quantum-chemistry calculations and show a systematic and
consistent improvement over standard exchange-correlation functionals---making
vdW-DF a promising candidate for resolving longstanding difficulties with
density functional theory in describing water. In addition, by comparing our
vdW-DF results with quantum-chemistry calculations and other standard
exchange-correlation functionals, we shed light on the question of why standard
functionals often fail to describe these systems accurately.
