# Non-Crossing Rule for Energy Curves in Diatomic Molecules

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Let and be energy curves for two different electronic states of a diatomic molecule, both computed within the Born–Oppenheimer approximation. If the two states belong to different symmetry species, say and , and , or singlet and triplet, there is no restriction on whether the curves can cross. If, however, the two states have the same symmetry, a non-crossing rule applies. Close approach of the two curves results in mutual repulsion, known as an anticrossing. For near degeneracy of and , a perturbation , representing higher-order contributions in the Born–Oppenheimer approximation, becomes significant, giving mixed states that do not cross.

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Contributed by: S. M. Blinder (March 2011)

Open content licensed under CC BY-NC-SA

## Snapshots

## Details

Snapshot 1: no restriction on crossing for states of different symmetry

Snapshot 2: geometry of an anticrossing

Snapshot 3: stronger interaction between states

Reference: L. D. Landau and E. M. Lifshitz, *Quantum Mechanics, Non-Relativistic Theory*, 2nd ed., Reading, MA: Addison–Wesley, 1965 p. 279.

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