  Using first-principles calculations based on density-functional theory (DFT)
we investigated the effects of the van der Waals (vdW) interactions on the
structural and electronic properties of anthracene and pentacene adsorbed on
the Ag(111) surface. We found that the inclusion of vdW corrections strongly
affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding
adsorption heights and energies more consistent with the experimental results
than standard DFT calculations with generalized gradient approximation (GGA).
For anthracene/Ag(111) the effect of the vdW interactions is even more
dramatic: we found that pure DFT-GGA calculations (without including vdW
corrections) result in preference for a tilted configuration, in contrast to
experimental observations of flat-lying adsorption; including vdW corrections,
on the other hand, alters the binding geometry of anthracene/Ag(111), favoring
the flat configuration. The electronic structure obtained using a
self-consistent vdW scheme was found to be nearly indistinguishable from the
conventional DFT electronic structure once the correct vdW geometry is employed
for these physisorbed systems. Moreover, we show that a vdW correction scheme
based on a hybrid functional DFT calculation (HSE) results in an improved
description of the highest occupied molecular level of the adsorbed molecules.
