  The interaction of water molecules with metal surfaces is typically weak and
as a result van der Waals (vdW) forces can be expected to be of importance.
Here we account for the systematic poor treatment of vdW forces in most popular
density functional theory (DFT) exchange-correlation functionals by applying
accurate non-local vdW density functionals. We have computed the adsorption of
a variety of exemplar systems including water monomer adsorption on Al(111),
Cu(111), Cu(110), Ru(0001), Rh(111), Pd(111), Ag(111), Pt(111), and
unreconstructed Au(111), and small clusters (up to 6 waters) on Cu(110). We
show that non-local correlations contribute substantially to the water-metal
bond in all systems, whilst water-water bonding is much less affected by
non-local correlations. Interestingly non-local correlations contribute more to
the adsorption of water on the reactive transition metal substrates than they
do on the noble metals. The relative stability, adsorption sites, and
adsorption geometries of competing water adstructures rarely differ when
comparing results obtained with semi-local functionals and the non-local vdW
density functionals, which explains the previous success of semi-local
functionals in characterizing adsorbed water structures on a number of metal
surfaces.
