The Marcus-Jortner-Levich model builds on classical Marcus theory, while considering the semiclassical form for the rate constant. It is able to incorporate quantum tunneling effects, and overcomes complications concerning the "inverted region" in Marcus theory by factoring in vibronic coupling between states within the harmonic approximation, and separates E λ into an inner sphere E λ,V and outer sphere E λ,S . In terms of a full active space treatment, whereby all modes are considered, can be cast as :