Reactant zero-point energy is needed to access the saddle point in molecular dynamics calculations of the association reaction H + C2D2 → C2D2H∗

We report classical (CT) and quasiclassical (QCT) trajectory calculations of the association reaction H + C2D2 → C2D2H∗, using an ab initio global potential energy surface. The QCT threshold is roughly 4 kcal/mol, in very good agreement with the vibrationally adiabatic ground-state barrier. In contrast, the 0 and 300 K CT threshold energies are roughly 10 and 8 kcal/mol, respectively. These contrasting results are analyzed in detail as functions of the impact parameter and collision energy, and are of potential significance for general molecular dynamics simulations, which typically do not include zero-point energy of the reactants.