  The dynamics of the reaction H + OH $\to$ O (3P) + H2 have been studied in a
series of quasi-classical trajectory (QCT) calculations and transition state
theory (TST) methods using high quality 3A' and 3A'' potential energy surfaces
(PESs). Accurate OH (v, j) state resolved cross sections and rate constants on
both potential energy surfaces are presented and fitted for OH at (v = 0, j =
0-16) and (v = 1, j = 0-6). The cross sections were calculated for different
collisional energies (Ec), ranging from the threshold energy at each specific
rovibrational state up to 1.0 eV with step sizes of 0.1 eV or less. They
increase steeply with collision energy when the barrier to reaction can be
overcome, after which the cross sections stay nearly constant with energy.
State resolved rate constants in the temperature range 200-2500 K are presented
based on the cross sections. Total thermal rate constants were calculated by
summing the rates for reaction on the 3A' and 3A'' potential energy surfaces
weighted by 1/3 and taking into account the thermal populations of the
rovibrational states of the OH molecules. It is shown that the improved
canonical variational transition (CVT) treatments with the approximation of
zero-curvature tunneling (ZCT) or small-curvature tunneling (SCT) produce
results more in accord with the QCT results than the TST and CVT methods. The
reactions are governed by the direct reaction mechanism. The rate constants for
OH in excited vibrational and rotational states are orders of magnitude larger
than the thermal rate constants, which needs to be taken into account in
astrochemical models.
