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The isomerization of hydrogen cyanide to hydrogen isocyanide on icy grainsurfaces is investigated by an accurate composite method (jun-Cheap) rooted inthe coupled cluster ansatz and by density functional approaches. Afterbenchmarking density functional predictions of both geometries and reactionenergies against jun-Cheap results for the relatively small model system HCN --(H2O)2 the best performing DFT methods are selected. A large cluster containing20 water molecules is then employed within a QM/QM$'$ approach to include arealistic environment mimicking the surface of icy grains. Our results indicatethat four water molecules are directly involved in a proton relay mechanism,which strongly reduces the activation energy with respect to the directhydrogen transfer occurring in the isolated molecule. Further extension of thesize of the cluster up to 192 water molecules in the framework of a three-layerQM/QM'/MM model has a negligible effect on the energy barrier ruling theisomerization. Computation of reaction rates by transition state theoryindicates that on icy surfaces the isomerization of HNC to HCN could occurquite easily even at low temperatures thanks to the reduced activation energythat can be effectively overcome by tunneling.

Gliding on ice in search of accurate and cost-effective computational  methods for astrochemistry on grains: the puzzling case of the HCN  isomerization