Complex-valued classical trajectories for reactive tunneling in three-dimensional collisions of H and H2

Complex‐valued classical trajectories for three‐dimensional reactive collisions of H+H₂ have been calculated at collision energies below the classical threshold for reaction, and from such trajectories classical S‐matrix elements for the $\text{0 → 1}$ rotational transition have been constructed. Comparison with available quantum mechanical results for the same system are encouraging and suggest that this semiclassical theory is capable of accurately describing reactive tunneling in a physically realistic model of a chemical reaction. Ways of simplifying the practical aspects of applying classical S‐matrix theory to three‐dimensional reactive systems are also described.