Imaad M. Ansari

Abstract The reactive singlet state of oxygen (O2) can decay to the triplet ground state nonradiatively in the presence of a solvent. There is a controversy about whether tunnelling is involved in this nonadiabatic spin-crossover process. Semiclassical instanton theory provides a reliable and practical computational method for elucidating the reaction mechanism and can account for nuclear quantum effects such as zero-point energy and multidimensional tunnelling.

1. Introduction The Born–Oppenheimer approximation is widely used to simplify the Schrödinger equation for molecular systems by expressing the equations of motion in terms of a single adiabatic potential-energy surface (PES). However, there are many molecular processes for which this adiabatic approximation is not valid, typically due to two PESs becoming close in energy. In these situations, alternative approaches are necessary.