Theoretical Calculations of the Thermal Rate Coefficients for the Interstellar NH3+ + H2 → NH4+ + H Reaction on a New Δ‑Machine Learning Potential Energy Surface

The NH3+ + H2 → NH4+ + H ion–molecule hydrogen-exchange reaction is important for understanding the mechanisms of ammonia formation in interstellar medium and circumstellar environments. The thermal rate coefficients of this reaction are calculated using the ring polymer molecular dynamics approach, which can account for the nuclear quantum effects, on a newly developed ab initio potential energy surface (PES). The PES was constructed by fitting a large number of ab initio points at the MP2/cc-pVDZ and CCSD(T)/cc-pVTZ levels using a Δ-machine learning approach. The obtained rate coefficients exhibited a minimum at T ∼ 130 K, and this characteristic temperature-dependent behavior qualitatively agreed with the experimental measurements. This study confirms that quantum tunneling plays a significant role in the reaction dynamics along with the existence of the metastable NH3+···H2 pre-reactive complex in the entrance channel of the PES.