When elementary reactive processes occur at so low energies that only few states of reactants and/or products are available, quantum effects strongly manifest and the standard description of the dynamics within the classical framework fails. We show here, for H 2 scattering on Pd(111), that by pseudo-quantizing in Bohr's spirit the relevant final actions of the system, along with adequately treating the diffractionmediated-trapping of the incoming wave, classical simulations get to an unprecedented and spectacular agreement with state-of-the-art quantum dynamics calculations.