Multidimensional Quantum Tunneling:
Direct Instanton Calculation with Application to Polyatomic Molecules

Quantum tunneling plays a crucial role at the nano scale.
Multidimensional tunneling appears in the study of many problems
ranging from quantum field to enzyme catalysis.
The high dimensionality of the potential energy surface
(e.g. many degrees of freedom) poses a great challenge in both
theoretical and numerical description of tunneling.
Numerical simulation based on Schrodinger equation is often
prohibitvely expensive. We propose an efficient and accurate
numerical method to calculate the ground state tunneling
splitting and decay rate.  The method is based on path integral formalism
('instanton' and 'bounce' approach) and free from any further
ad hoc assumptions on potential energy surface.
The application to proton tunneling between isomers of
polyatomic molecule is demonstrated.