Improved dual-level direct dynamics method for reaction rate calculations with inclusion of multidimensional tunneling effects and validation for the reaction of H with trans-N₂H₂

A new scheme for carrying out dual-level direct dynamics calculations is presented in this paper. A better estimate of the barrier width is obtained by using the high-level imaginary frequency at the saddle point as well as high-level values of the energies of three stationary points (i.e., reactants, products, and saddle point). Furthermore, a more robust formula is introduced for incorporating high-level vibrational frequency corrections on the generalized normal modes along the reaction path. Incorporating these improvements, we carry out dual-level calculations of the reaction rate of H + N₂H₂ → H₂ + N₂H by employing variational transition-state theory with optimized multidimensional tunneling. Dual-level calculations at the level of zero-curvature tunneling (ZCT) show excellent agreement with an earlier calculation involving high-level computations at 11 times as many geometries. Having validated the dual-level approach at the ZCT level, we next extend the dual-level calculations to include small-curvature, large-curvature, and optimized multidimensional tunneling approximations. Four choices of low-level surface are used to gauge the sensitivity to these choices.