Within the Born–Oppenheimer approximation, a semi-classical model for the @P04855@, \(P\), of hopping from one @ET07026@ to another of the same or different 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 @S06166@ 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.