We investigated the phenomenon of atom tunneling in chemical reactions using several approaches. We developed a method to simulate tunneling rate constants with higher accuracy and lower computational demand than previously possible. This was achieved by deriving a microcanonical formulation of instanton theory along with an extension of instanton theory to higher temperatures. This allows the calculation of bimolecular rate constants over the whole temperature range over a large range of pressures. The computational efficiency was increased by a dual-level scheme for instanton theory, as well as by employing machine-learning techniques for the approximation of potential energy surfaces. To treat surface reactions, especially in astrochemistry, we found an implicit surface treatment, which allows us to calculate surface rate constants at the computational cost of gas-phase reactions.
We applied it to many reactions relevant to the interstellar medium, gas-phase reactions as well as surface reactions. The resulting rate constants were subsequently used in astrochemical models and resulted in an improved agreement with observed abundances. The tunnel effect is also important in biochemical systems (enzymes). We investigated the enzyme Taurine/alpha-ketoglutarate Dioxygenase (TauD). Moreover, we have written a review paper on atom tunneling in chemistry and edited a textbook on the subject to be published by the RSC to serve as a general source of reference for atom tunneling in chemistry.