Theory and simulation of atom tunneling in chemical reactions.

Quantum tunneling of atoms, the penetration of energy barriers higher than the total energy of the system, plays a role in many chemical systems. While any chemical reaction is dominated by tunneling at low enough temperature, there is evidence for hydrogen atom tunneling even in enzymatic reactions at ambient conditions. The smaller the mass of the atoms, the lower and thinner the barrier is, the stronger the tunneling effect increases the reaction rate. Different methods to calculate tunneling rates are available. They range from full solutions of the time-dependent Schrödinger equation via the semiclassical method to ad hoc corrections of classical transition state theory. The basis of different methods, their accuracy, and applicability is discussed in the present overview, with a particular focus on instanton theory, a Feynman-path-based approach using the semiclassical approximation. [ABSTRACT FROM AUTHOR]