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.

128Highlights► We report trajectory calculations of the association probability for H + C2D2 → C2D2H∗. ► The classical threshold energy is higher than the barrier height to form C2D2H. ► The quasiclassical threshold energy is close to the barrier height. ► We analyze the source of the surprisingly high threshold in the classical results. ► A general deficiency of classical trajectory method was indicated by our analysis.