Electronic states of the same symmetry cannot cross along a reaction coordinate.
Note:
Only applies to diatomic molecules and not to polyatomic molecules.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 374 (https://doi.org/10.1351/pac200779030293)