Within the Born–Oppenheimer approximation, a semi-classical model for the probability,
,
of hopping from one electronic state to another of the same or different *multiplicity*
where
is the potential energy gap between the two electronic states at a surface crossing point,
is the difference in slopes between the intersecting potential energy curves at this
point and
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