INTEGRAL CROSS-SECTIONS FOR H + H-2(0,0)-]H-2(0,J') + H - COMPARISON OF QUASI-CLASSICAL AND QUANTUM RESULTS

State-to-state integral cross sections for the exchange reaction H + H-2(0,0) --> H-2(0,j') + H have been calculated with quasiclassical trajectories (QCT) on the LSTH potential energy surface. Microscopic reversibility permits an alternative, indirect, means of calculating cross sections based on trajectories starting in the (0,j') state in addition to the usual direct one based on trajectories starting in (0,0). These QCT cross sections are compared with one another and with recent exact quantum calculations at relatively low total energy (approximately 1 eV). We identify some systematic effects relating to the exchange barrier in the potential, which is important for defining the threshold for low j'. Some differences between direct and indirect cross sections close to threshold arise from the binning procedure by which the outcomes of quasiclassical trajectories are assigned to quantum states. Overall we find that indirect QCT agrees best with the quantum results near threshold and that the agreement improves as j' increases. Therefore, at least for this particular reason, we recommend the use of indirect QCT when quantum calculations are unavailable or intractable. Some additional QCT cross sections were calculated by using the DMBE surface. The relative agreement between QCT and quantum calculations persists. Differences between the DMBE and LSTH results seen in both QCT and quantum methods indicate rather similar tunneling contributions in the energy range available for comparison.