non-crossing rule

https://doi.org/10.1351/goldbook.NT07425
Electronic states of the same symmetry cannot cross along a @R05168@.
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 [Terms] [Paper]