  Two different adsorption configurations of benzene on the Si(001)-(2x1)
surface, the tight-bridge and butterfly structures, were studied using density
functional theory. Several exchange and correlation functionals were used,
including the recently developed vdW-DF functional, which accounts for the
effect of van der Waals forces. In contrast to the PBE, revPBE and other GGA
functionals, the vdW-DF functional finds that, for most coverages, the
adsorption energy of the butterfly structure is greater than that of the
tight-bridge structure.
