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
is the potential energy gap between the two electronic states at a surface crossing
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.
The original formalism only considered states of the same spin multiplicity.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 362
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook