Variational transition-state theory calculation using the direct dynamics method: NH3+H→NH2+H2 reaction

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Abstract
The title reaction has been studied following the ‘‘direct dynamics’’ method, which uses information on electronic structure energy and energy derivatives calculated ab initio along the minimum energy path. With this information, the rate constants were calculated for the temperature range 200–2000 K, using the variational transition‐state theory and different approximations to calculate the tunneling effect. With respect to the construction of the minimum energy path, some possible sources of error were analyzed and quantified, namely, the number of points calculated along the minimum energy path (MEP), and the scale factor in the Hessian matrix. With respect to the comparison with experimental values, several factors were also analyzed, namely, the correlation energy, the influence of spin projection and the basis set superposition error on the height of the barrier, the influence of anharmonicity on the final rate constants, and different ways to scale the potential energy curve. The conclusion was that the variational effects are small for this reaction, and for T≥500 K the tunneling effect is also small. Good agreement was found with the experimental values when using the PMP‐SAC4/A1 curve and the Hessian matrix scaled by 0.9.