Influence of van der Waals forces on the adsorption structure of benzene   on silicon
  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. 
Karen Johnston, Jesper Kleis, Bengt I. Lundqvist and Risto M. Nieminen