Adiabatic and non-adiabatic dynamics studies of O(¹D) + H₂ → OH + H

This paper presents a detailed study of the title reaction, with emphasis on the contribution of excited electronic states, and on coupling between ground and excited states, in the reaction dynamics. Most of the calculations are based on diatomics-in-molecules (DIM) potential energy surfaces and couplings, but to provide calibration we compare adiabatic DIM results for the two lowest surfaces with corresponding results for high quality ab initio surfaces that were developed using the reproducing kernel Hubert space interpolation method. These comparisons indicate that the DIM surfaces are close to being correct, with the most serious deficiencies occurring in the long range behavior of the 1A′ surface (too attractive), and the barrier height of the 1A″ surface (too high). The effect of non-adiabatic coupling has been described using Tully's fewest switches method and the DIM surfaces and couplings. We find that the surface hopping trajectories for collisions starting on the ground (1A′) surface show slightly colder product vibrations and rotations, and less backscattering than the equivalent adiabatic calculations, while the overall integral cross-sections are not significantly changed. For trajectories that start on the 2A′ surface, where reaction cannot occur in the absence of surface hopping, we find that the cross-section is often about half that associated with the 1A″ surface (which has the same collinear barrier but does not require hopping for reaction to occur). Product state distributions on the 2A′ surface are intermediate in character between 1A′ and 1A″.