Exact activation energies and phenomenological description of quantum tunneling for model potential energy surfaces. The FÂ +Â H2 reaction at low temperature

► Exact quantum mechanical rate constants presented for the F + H2 reaction. ► Deviation from linearity found for Arrhenius rates in the tunneling regime. ► Effective activation energies characterized for potential barriers of different width. ► Phenomenological descriptions for observed curvature in Arrhenius plots. ► The recommended rate law involves a deformation of the exponential function.