  We report on a theoretical study of adsorption of 4-Acetylbiphenyl molecule
and its diffusion properties in the main directions of the Au(111) surface.
Structural changes of the molecule, which are induced by adsorption lead to
stronger conjugation of the $\pi$-system. The molecule is adsorbed in a flat
configuration on the surface with roughly the same binding energy along the
[110] and [112] directions, in good agreement with experiments. Furthermore,
the diffusion barriers imply an important directionality of the
molecule-surface interactions. This is somewhat surprising because our
calculations show that the prevailing interaction is the long-range
molecule-surface van der Waals interaction. Despite of its weakness, the van
der Waals interaction discriminates the preferential adsorption sites as well
as imposes a molecular geometry that needs to be considered when rationalizing
the diffusion barriers.
