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

Within the Born–Oppenheimer approximation, a semi-classical model for the @[email protected], \(P\), of hopping from one @[email protected] to another of the same or different

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

*multiplicity*\[P=\exp (- \frac{4\ \pi ^{2}\ \varepsilon_{12}^{2}}{h\ \nu\ |s_{1}-s_{2}|})\] where \(ɛ_{12}\) is the potential energy gap between the two electronic states at a @[email protected] point, \(|s_{1}- s_{2}|\) is the difference in slopes between the intersecting potential energy curves at this point and \(\nu\) 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.