  The structures and energetics of organic molecules adsorbed onto clean and
H-passivated Si(001)-(2$\times$1) surfaces have been calculated using density
functional theory. For benzene adsorbed on the clean Si surface the
tight-bridge structure was found to be stable and the butterfly structure
metastable. Both carbonic acid H$_2$CO$_3$ and propane C$_3$H$_8$ dissociate on
contact with the surface. Passivation of the Si surface with H-atoms has a
dramatic effect on the surface properties. The passivated surface is very inert
and the binding energy of all the molecules is very weak.
