  We present Car-Parrinello molecular dynamics simulations of the initial
reaction steps leading to an inert oxide layer on aluminium. The mechanism of
the reaction of the aluminium surface with single oxygen molecules is analysed.
After adsorption at the surface the oxygen molecules dissociate at a
femtosecond timescale and the atoms are chemisorbed at the surface at a
distance of several angstrom. When the aluminium surface is exposed to higher
oxygen pressure, a surface layer essentially consisting of threefold
coordinated oxygen atoms starts to form.
