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.