  We propose a second version of the van der Waals density functional (vdW-DF2)
of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate
semilocal exchange functional and the use of a large-N asymptote gradient
correction in determining the vdW kernel. The predicted binding energy,
equilibrium separation, and potential-energy curve shape are close to those of
accurate quantum chemical calculations on 22 duplexes. We anticipate the
enabling of chemically accurate calculations in sparse materials of importance
for condensed-matter, surface, chemical, and biological physics.
