  The adsorption of benzene on the Si(100) surface is studied theoretically
using the self-consistent van der Waals density functional (vdW-DF) method. The
adsorption energies of two competing adsorption structures, butterfly (BF) and
tight-bridge (TB) structures, are calculated with several vdW-DFs at saturation
coverage. Our results show that recently proposed vdW-DFs with high accuracy
all prefer TB to BF, in accord with more accurate calculations based on exact
exchange and correlation within the random phase approximation. Detailed
analyses reveal the important roles played by the molecule-surface interaction
and molecular deformation upon adsorption, and we suggest that their precise
description is prerequisite for accurate prediction of the most stable
adsorption structure of organic molecules on semiconductor surfaces.
