https://doi.org/10.1351/goldbook.LT07409

Within the Born–Oppenheimer approximation, a semi-classical model for the probability, P, of hopping from one electronic state to another of the same or different *multiplicity* \[P=\exp (- \frac{4\ \pi ^{2}\ \varepsilon_{12}^{2}}{h\ \nu\ |s_{1}-s_{2}|})\] where ɛ12 is the potential energy gap between the two electronic states at a surface crossing point, |s1- s2| is the difference in slopes between the intersecting potential energy curves at this point and ν is the nuclear relative velocity with which the system passes the point of closest approach.**Note: **

The original formalism only considered states of the same spin multiplicity.*Source: *

PAC, 2007,*79*, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 362 (https://doi.org/10.1351/pac200779030293)

The original formalism only considered states of the same spin multiplicity.

PAC, 2007,