Frequently, two Born–Oppenheimer electronic states (A, B) change their energy order
as molecular geometry (x
) is changed continuously along a path. In the process their energies may become equal
at some points (the surfaces are said to cross, dotted lines in the figure), or only
come relatively close (the crossing of the surfaces is said to be avoided). If the
electronic states are of the same symmetry, the surface crossing
is always avoided in diatomics and usually avoided in polyatomics.
Synonymous with intended crossing
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2229
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. https://doi.org/10.1351/goldbook