  Quantum mechanical tunneling of atoms is increasingly found to play an
important role in many chemical transformations. Experimentally, atom-tunneling
can be indirectly detected by temperature-independent rate constants at low
temperature or by enhanced kinetic isotope effects. On the contrary, using
computational investigations the influence of tunneling on the reaction rates
can directly be monitored. The tunnel effect, for example, changes reaction
paths and branching ratios, enables chemical reactions in an astrochemical
environment that would be impossible by thermal transition, and influences
biochemical processes.
