A wave packet 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 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 the H-2 vibration, which is much more excited than the substrate. Focusing on the surface relaxation effects, the results are compared with two widely used approximations, namely, the sudden approximation and the adiabatic approximation.