  The interaction of hydrogen with many transition metal surfaces is
characterized by a coexistence of activated with non-activated paths to
adsorption with a broad distribution of barrier heights. By performing
six-dimensional quantum dynamical and classical molecular dynamics calculations
using the same potential energy surface derived from ``ab initio'' calculations
for the system H_2/Pd(100) we show that these features of the potential energy
surface lead to strong steering effects in the dissociative adsorption
dynamics. The adsorption dynamics shows only a small isotope effect which is
purely due to the quantum nature of hydrogen.
