A wavepacket study of the dynamics of H 2 formation through a collinear Eley Rideal mechanism that explicitly handles the substrate relaxation effects is presented. The substrate used is a planar polycyclic aromatic hydrocarbon (PAH) which exhibits some strong similarities with a perfect graphite (0001) surface. The collision energy range investigated lies between 0.4 meV (T=5 K) and 0.46 eV. The reaction probability is large, except at very low collision energy. Most of the available energy goes into H 2 vibration. The substrate relaxation facilitates the reaction.