We present an analysis of the chemistry of dissociative adsorption bas
ed on an ab initio molecular dynamics simulation of Cl2 incident on Si
(111)-2x1 at an energy of 1 eV. It is shown that the break-up of the m
olecule occurs because of charge transfer into a more antibonding mole
cular orbital. The process is strongly orientationally dependent and i
s absent if the molecule is oriented perpendicular to the surface. In
this case the molecule enters a precursor state and subsequently disso
ciates when it escapes from the local energy minimum.