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On the surface of icy dust grains in the dense regions of the interstellarmedium a rich chemistry can take place. Due to the low temperature, reactionsthat proceed via a barrier can only take place through tunneling. The reactionH + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH is such a case with a gas-phasebarrier of $\sim$26.5 kJ/mol. Still the reaction is known to be involved inwater formation on interstellar grains. Here, we investigate the influence of awater ice surface and of bulk ice on the reaction rate constant. Rate constantsare calculated using instanton theory down to 74 K. The ice is taken intoaccount via multiscale modeling, describing the reactants and the directsurrounding at the quantum mechanical level with density functional theory(DFT), while the rest of the ice is modeled on the molecular mechanical levelwith a force field. We find that H$_2$O$_2$ binding energies cannot be capturedby a single value, but rather depend on the number of hydrogen bonds withsurface molecules. In highly amorphous surroundings the binding site can blockthe routes of attack and impede the reaction. Furthermore, the activationenergies do not correlate with the binding energies of the same sites. Theunimolecular rate constants related to the Langmuir-Hinshelwood mechanismincrease as the activation energy decreases. Thus, we provide a lower limit forthe rate constant and argue that rate constants can have values up to two orderof magnitude larger than this limit.

Influence of surface and bulk water ice on the reactivity of a water-forming reaction