Tunneling Reaction Kinetics for the Hydrogen Abstraction Reaction H +
  H$_2$S -> H$_2$ + HS in the Interstellar Medium
  The hydrogen abstraction reaction between H and H$_2$S, yielding HS and H$_2$
as products, has been studied within the framework of interstellar surface
chemistry. High-temperature rate constants up to 2000 K are calculated in the
gas phase and are in agreement with previously reported values. Subsequently
low-temperature rate constants down to 55 K are presented for the first time
that are of interest to astrochemistry, i.e., covering both bimolecular and
unimolecular reaction mechanisms. For this, a so-called implicit surface model
is used. Strictly speaking, this is a structural gas-phase model in which the
restriction of the rotation in the solid state is taken into account. The
calculated kinetic isotope effects are explained in terms of difference in
activation and delocalization. All rate constants are calculated at the
UCCSD(T)-F12/cc-VTZ-F12 level of theory. Finally, we show that the energetics
of the reaction is only affected to a small extent by the presence of H$_2$O or
H$_2$S molecular clusters that simulate an ice surface, calculated at the
MPWB1K/def2-TZVP level of theory.
