  We report the first six-dimensional quantum dynamical calculations of
dissociative adsorption and associative desorption. Using a potential energy
surface obtained by density functional theory calculations, we show that the
initial decrease of the sticking probability with increasing kinetic energy in
the system H_2/Pd(100), which is usually attributed to the existence of a
molecular adsorption state, is due to dynamical steering. In addition, we
examine the influence of rotational motion and orientation of the hydrogen
molecule on adsorption and desorption.
