The results of an accurate quantum mechanical treatment of the planar H+H_2 exchange reaction on a realistic potential energy surface are presented. Full vibration–rotation convergence was achieved in the calculations, and this, together with a large number of auxiliary convergence and invariance tests, indicates that the cross sections are accurate to 5% or better. The reactive differential cross sections are always backward peaked over the range of total energies from 0.3 to 0.65 eV. Nonreactive j=0 to j′=2 cross sections are backward peaked at low energy (0.4 eV) shifting to sidewards peaking for E≳0.5 eV. Quantum symmetry interference oscillations are very significant in the j=0 to j′=2 para‐to‐para cross sections for E≥0.6 eV. Reactive integral cross sections show two distinct kinds of energy dependence. At low energy (<0.5 eV), barrier tunneling gives them a largely exponential energy dependence while above 0.5 eV (the effective threshold energy) the cross sections vary nearly linearly. Comparison of collinear and coplanar transition probabilities indicates similar 1D and 2D energy dependence but with a shift in energy from 1D to 2D due to bending motions in the transition state. An analysis of rotational distributions indicates surprisingly good correspondence with temperaturelike distributions. The results of a one‐vibration‐approximation calculation are examined, and errors of as much as three orders of magnitude are found at some energies. Shapes of angular distributions are, however, accurately predicted by this approximate method. Additional analyses include comparisons with previous distorted wave and coupled‐channel results, and calculations of thermal rate constants
