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Reaction probabilities as a function of total angular momentum and theresulting reaction cross-sections for the collision of open shell S($^1$D)atoms with para-hydrogen have been calculated in the kinetic energy range0.09--10 meV (1--120 K). The quantum mechanical (QM) hyperspherical reactivescattering method and quasi--classical trajectory (QCT) and statisticalquasiclassical trajectory (SQCT) approaches were used. Two different ab initiopotential energy surfaces (PESs) have been considered. The widely used RKHS PESby Ho et al. (J. Chem. Phys. 116, 4124, 2002) and the recently publishedDMBE/CBS PES by Song and Varandas (J. Chem. Phys. 130, 134317, 2009). Thecalculations at low collision energies reveal very different dynamicalbehaviors on the two PESs. The reactivity on the RKHS PES is found to beconsiderably larger than that on the DMBE/CBS PES. The observed differenceshave their origin in two major distinct topography features. Although both PESsare essentially barrierless for equilibrium H--H distances, when the H--H bondis compressed the DMBE/CBS PES gives rise to a dynamical barrier which limitsthe reactivity of the system. This barrier is completely absent in the RHKSPES. In addition, the latter PES exhibits a van der Walls well in the entrancechannel which reduces the height of the centrifugal barrier and is able tosupport resonances. As a result a significant larger cross section is found onthis PES, with marked oscillations attributable to shape resonances and/or tothe opening of partial wave contributions.

On the role of dynamical barriers in barrierless-reactions at low energies: S($^1$D) + H$_2$