## Landau–Zener model

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 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.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 362 [Terms] [Paper]