Non-born–oppenheimer molecular dynamics for conical intersections, avoided crossings, and weak interactions

Processes involving nonradiative transitions between electronic states are ubiquitous in chemistry — from spin-forbidden reactions in combustion to light harvesting in solar cells — and they occur via a variety of elementary chemical mechanisms, such as intersystem crossing, internal conversion, and nonadiabatic electron transfer. The term “non-Born–Oppenheimer” (NBO) may be generally applied to these processes to emphasize the idea that the Born–Oppenheimer separation of the nuclear and electronic time scales breaks down and that potential energy surfaces other than the ground-electronic-state adiabatic potential energy surface play a role in the dynamics. A detailed understanding of NBO coupling of adiabatic electronic states and of the potential energy surfaces associated with them and the ability to predict the effect of this kind of coupling for real chemical systems remain significant challenges to current theories.