  The non-local van der Waals density functional (vdW-DF) of Dion et al. [Phys.
Rev. Lett. 92, 246401 (2004)] is a very promising scheme for the efficient
treatment of dispersion bonded systems. We show here that the accuracy of
vdW-DF can be dramatically improved both for dispersion and hydrogen bonded
complexes through the judicious selection of its underlying exchange
functional. New and published exchange functionals are identified that deliver
much better than chemical accuracy from vdW-DF for the S22 benchmark set of
weakly interacting dimers and for water clusters. Improved performance for the
adsorption of water on salt is also obtained.
