We report the first simulation of a surface chemical reaction performe
d with the ab initio molecular dynamics approach. A set of trajectorie
s with different initial conditions has been generated for single Cl2
molecules impinging on the Si(111)-2 x 1 surface with incident transla
tional energy of 1 eV. We observe a high probability of dissociation,
triggered by active sites on the pi-bonded chains, and accompanied by
a large surface response and local rehybridization effects.