The darkness readily observed between the stars on a clear night is far from empty. The low temperatures (T≈10 K) in dark molecular clouds combined with the low particle densities (n = 10^4 cm−3) make it seem unlikely for chemistry to take place efficiently. The chemistry that occurs can be partly explained by the presence of ice-coated dust grains on which molecules freeze out and, when they find each other, react. These grains act both as a molecule reservoir and as an energy sink for exothermic reactions. As such they allow a rich chemistry to occur.

Barriers at cryogenic temperatures can only be overcome, however, when tunneling is invoked as a crucial component of the reaction mechanism. Hydrogen is very abundant in the interstellar medium and many surface reactions involve H transfer reactions, ultimately leading to the formation of saturated species like H2O and C2H6 [1]. Furthermore, tunneling can also affect the D/H ratio of the products that are finally formed [2].

Rate constants for tunneled reactions are calculated with the use of instanton theory, while the ice surface is taken into account via, e.g., small clusters or multiscale modeling approaches (QM/MM). We elaborate on the influence of an ice environment on reaction rate constants, how and when an ice can be approximated without taking into account all degrees of freedom, and how the rate constants are to be interpreted in the light of astrochemical mean-field models and observations.