  Here we a present a density functional theory study of pentacene ultra-thin
films on flat [(111)] and vicinal [(455)] Au surfaces. We have performed
crystal and electronic structure calculations by using PBE and optB86b-vdW
functionals and investigated the effects of long range Van der Waals
interactions for different coverages starting from a single isolated molecule
up to 4 monolayers of coverage. For an isolated molecule both functionals yield
the hollow site as the most stable one with bridge-60 site being very close in
energy in case of optB86b-vdW. Binding strength of an isolated pentacene on the
step edge was found to be much larger than that on the terrace sites. Different
experimentally reported monolayer structures were compared and the (6 x 3) unit
cell was found to be energetically more stable than the (2 x 2 $\sqrt{7}$) and
(2 x $\sqrt{31}$) ones. For one monolayer films while dispersion corrected
calculations favored flat pentacene molecules on terraces, standard (PBE)
calculations either found tilted and flat configurations to be energetically
similar (on (111) surface) or favored the tilted configuration (on (455)
surface). PDOS calculations performed with optB86b-vdW functional showed larger
dispersion of molecular orbitals over the Au states for the (455) surface when
compared with the (111) surface, indicating an enhanced charge carrier
transport at the pentacene-gold interface in favor of the vicinal surface.
Starting with the second monolayer, both functionals favored tilted
configurations for both surfaces. Our results underline the importance of the
dispersion corrections for the loosely bound systems like pentacene on gold and
the role played by step edges in determining the multilayer film structure and
charge transfer at the organic molecule-metal interface.
