  The modification of the potential-energy surface (PES) of H_2 dissociation
over Pd(100) as induced by the presence of a (2x2) S adlayer is investigated by
density-functional theory and the linear augmented plane wave method. It is
shown that the poisoning effect of S originates from the formation of energy
barriers hindering the dissociation of H_2. The barriers are in the entrance
channel of the PES and their magnitude strongly depends on the lateral distance
of the H_2 molecule from the S adatoms.
