Fully converged integral cross sections of diatom-diatom reactions and the accuracy of the centrifugal sudden approximation in the H2+OH reaction

The initial state selected time-dependent wave packet method has been extended to calculate integral cross sections for diatom-diatom chemical reactions without the CS (centrifugal sudden) approximation by including all important K (the projection of the total angular momentum on the body-fixed axis) blocks in the body-fixed frame. We report the first fully converged cross section for the ground rovibrational state of the title reaction and present a detail study of the accuracy of the CS approximation to the reaction. We find that for the ground rovibrational state the CS approximation works very well, but its accuracy deteriorates with increasing reagent rotational excitation. As expected, and as found in atom-diatom reactions, the CS approximation works much better in high energy region than in low energy region. In low energy region, the coupled channel cross sections are larger than the CS ones for all the rotationally excited states investigated here, in particular for the highly excited states. It is found the CS approximation gives rise to about 10% error in ${\mathrm{H}}_{\mathrm{2}}$ or OH rotationally averaged rate constant. If simultaneous OH and ${\mathrm{H}}_2$ rotational excitation does not have a correlated effect on dynamics, the CS approximation introduces about 19% error in thermal rate constant for the reaction for low temperatures which is considerably larger than what is expected of a few percent.