  Adsorption of benzene on the Si(100) surface is studied from first
principles. We find that the most stable configuration is a
tetra-$\sigma$-bonded structure characterized by one C-C double bond and four
C-Si bonds. A similar structure, obtained by rotating the benzene molecule by
90 degrees, lies slightly higher in energy. However, rather narrow wells on the
potential energy surface characterize these adsorption configurations. A
benzene molecule impinging on the Si surface is most likely to be adsorbed in
one of three different di-$\sigma$-bonded, metastable structures, characterized
by two C-Si bonds, and eventually converts into the lowest-energy
configurations. These results are consistent with recent experiments.
