A general non-adiabatic quantum instanton approximation

Area of Science:

• Quantum chemistry
• Chemical kinetics
• Theoretical chemistry

Background:

• Calculating reaction rates is crucial for understanding chemical processes.
• Existing methods have limitations for systems with strong electronic coupling.

Purpose of the Study:

• To introduce a general quantum instanton approach for calculating reaction rates.
• To develop the non-adiabatic quantum instanton (NAQI) approximation.

Main Methods:

• The NAQI approximation utilizes a saddle point approximation for the time integral of a reactive flux autocorrelation function.
• It employs a generalized definition of the projection operator onto product states.
• The method is validated against exact rates for one-dimensional scattering problems.

Main Results:

• The NAQI approximation unifies existing theories, reducing to Wolynes theory in the golden rule limit and a projected quantum instanton method in the adiabatic limit.
• Demonstrated accuracy for one-dimensional scattering problems.

Conclusions:

• The NAQI approximation provides a versatile and accurate tool for calculating reaction rates in systems with two electronic states.
• The approach is applicable to a range of complex chemical reactions.