Calculation of ion–ion mutual neutralization rate constants using Landau–Zener theory coupled with trajectory simulations for Ar⁺–Cl⁻, Br⁻, I⁻.

This article presents a computational study focused on calculating mutual neutralization (MN) rate constants for rare gas cation–halide anion pairs—specifically Ar⁺–Cl⁻, Ar⁺–Br⁻, and Ar⁺–I⁻—at a pressure of approximately 1 Torr (133 Pa). The authors employ multi-reference configuration interaction with Davidson correction (MRCISD+Q) and explicitly include spin–orbit coupling effects via the Breit–Pauli Hamiltonian to improve the accuracy of potential energy curves (PECs). These PECs provide inputs for the Landau–Zener (LZ) theory, which estimates electron transfer probabilities incorporated into classical Langevin dynamics simulations to predict MN rate constants. The computed rate constants show good agreement with experimental data, differing by factors of about 1.2 to 2, demonstrating the effectiveness of combining advanced electronic structure calculations with semi-classical trajectory simulations for modeling MN reactions under finite gas pressures.