Reaction Dynamics of the H + HeH$^+$ $\rightarrow$ He + H$_2^+$ System

Abstract: The reaction dynamics for the H + HeH$^+$ $\rightarrow$ He + H$2^+$ reaction in its electronic ground state is investigated using two different representations of the potential energy surface (PES). The first uses a combined kernel and neural network representation of UCCSD(T) reference data whereas the second is a corrected PES (cR-PES) that eliminates an artificial barrier in the entrance channel appearing in its initial expansion based on full configuration interaction reference data. Despite the differences between the two PESs, both yield $k{v=0,j=0} \approx 2 \times 10^{-9}$ cm$^3$/molecule/s at $T = 10$ K which is consistent with a $T-$independent Langevin rate $k_{\rm L} = 2.1 \times 10^{-9}$ cm$^3$/molecule/s but considerably larger than the only experimentally reported value $k_{\rm ICR} = (9.1 \pm 2.5) \times 10^{-10}$ cm$^3$/molecule/s from ion cyclotron resonance experiments. Similarly, branching ratios for the reaction outcomes are comparable for the two PESs. However, when analysing less averaged properties such as initial state-selected $T-$dependent rate coefficients and final vibrational states of the H$_2^+$ product for low temperatures, the differences in the two PESs manifest themselves in the observables. Thus, depending on the property analyzed, accurate and globally valid representations of the PES are required, whereas more approximate and empirical construction schemes can be followed for state-averaged observables.