Communications: Classical trajectory study of the postquenching dynamics of OH A 2Sigma+ by H2 initiated at conical...

We report results of a classical trajectory calculation of the postquenching dynamics of OH A (2)Sigma(+) by H(2). This is done by performing roughly 100 000 trajectories at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" model of the dynamics, respectively. The trajectories are propagated on a new ground state ab initio-based potential energy surface. This surface is a permutationally invariant fit to roughly 23,000 electronic energies (multireference configuration interaction/correlation-consistent-augmented-triple-zeta basis) at configurations obtained mostly from direct-dynamics calculations (complete active space second order perturbation theory with correlation-consistent-augment double-zeta basis), also initiated at the CoIs. Final rovibrational state distributions of the ground electronic state OH product and the H-atom translational energy distributions for abstraction and insertion mechanisms are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The calculated H(2) rovibrational distributions (not measured experimentally) are also presented.