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.
doi:10.1351/goldbook