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The final step of the water formation network on interstellar grain surfacesstarting from the H + O$_2$ route is the reaction between H and H$_2$O$_2$.This reaction is known to have a high activation energy and therefore at lowtemperatures it can only proceed via tunneling. To date, however, no rateconstants are available at temperatures below 200 K. In this work, we useinstanton theory to compute rate constants for the title reaction with andwithout isotopic substitutions down to temperatures of 50 K. The calculationsare based on density functional theory, with additional benchmarks for theactivation energy using unrestricted single-reference and multireferencecoupled-cluster single-point energies. Gas-phase bimolecular rate constants arecalculated and compared with available experimental data not only for H +H$_2$O$_2$ $\rightarrow$ H$_2$O + OH, but also for H + H$_2$O$_2$ $\rightarrow$H$_2$ + HO$_2$. We find a branching ratio where the title reaction is favoredby at least two orders of magnitude at 114 K. In the interstellar medium thisreaction predominantly occurs on water surfaces, which increases theprobability that the two reactants meet. To mimic this one, two, or threespectator H2O molecules are added to the system. Eley-Rideal bimolecular andLangmuir-Hinshelwood unimolecular rate constants are presented here. Thekinetic isotope effects for the various cases are compared to experimental dataas well as to expressions commonly used in astrochemical models. Both therectangular barrier and the Eckart approximations lead to errors of about anorder of magnitude. Finally, fits of the rate constants are provided as inputfor astrochemical models.

Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH