  The isomerization of hydrogen cyanide to hydrogen isocyanide on icy grain
surfaces is investigated by an accurate composite method (jun-Cheap) rooted in
the coupled cluster ansatz and by density functional approaches. After
benchmarking density functional predictions of both geometries and reaction
energies against jun-Cheap results for the relatively small model system HCN --
(H2O)2 the best performing DFT methods are selected. A large cluster containing
20 water molecules is then employed within a QM/QM$'$ approach to include a
realistic environment mimicking the surface of icy grains. Our results indicate
that four water molecules are directly involved in a proton relay mechanism,
which strongly reduces the activation energy with respect to the direct
hydrogen transfer occurring in the isolated molecule. Further extension of the
size of the cluster up to 192 water molecules in the framework of a three-layer
QM/QM'/MM model has a negligible effect on the energy barrier ruling the
isomerization. Computation of reaction rates by transition state theory
indicates that on icy surfaces the isomerization of HNC to HCN could occur
quite easily even at low temperatures thanks to the reduced activation energy
that can be effectively overcome by tunneling.
