Nonadiabatic transition state theory (NA-TST) is a powerful tool to predict rates of chemical reactions from a computational standpoint.
[1] In general, all of the assumptions taking place in traditional transition state theory (TST) are also used in NA-TST but with some corrections.
First, a spin-forbidden reaction proceeds through the minimum energy crossing point (MECP) rather than through transition state (TS).
[3] At this stage non-relativistic couplings responsible for mixing between states is a driving force of transition.
NA-TST can be reduced to the traditional TST in the limit of unit probability.