  Reaction probabilities as a function of total angular momentum and the
resulting reaction cross-sections for the collision of open shell S($^1$D)
atoms with para-hydrogen have been calculated in the kinetic energy range
0.09--10 meV (1--120 K). The quantum mechanical (QM) hyperspherical reactive
scattering method and quasi--classical trajectory (QCT) and statistical
quasiclassical trajectory (SQCT) approaches were used. Two different ab initio
potential energy surfaces (PESs) have been considered. The widely used RKHS PES
by Ho et al. (J. Chem. Phys. 116, 4124, 2002) and the recently published
DMBE/CBS PES by Song and Varandas (J. Chem. Phys. 130, 134317, 2009). The
calculations at low collision energies reveal very different dynamical
behaviors on the two PESs. The reactivity on the RKHS PES is found to be
considerably larger than that on the DMBE/CBS PES. The observed differences
have their origin in two major distinct topography features. Although both PESs
are essentially barrierless for equilibrium H--H distances, when the H--H bond
is compressed the DMBE/CBS PES gives rise to a dynamical barrier which limits
the reactivity of the system. This barrier is completely absent in the RHKS
PES. In addition, the latter PES exhibits a van der Walls well in the entrance
channel which reduces the height of the centrifugal barrier and is able to
support resonances. As a result a significant larger cross section is found on
this PES, with marked oscillations attributable to shape resonances and/or to
the opening of partial wave contributions.
