For what elements does Born–Oppenheimer approximation fail the most?

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SUMMARY

The Born–Oppenheimer (BO) approximation primarily fails for elements with low atomic numbers due to the significant influence of their lighter nuclei on electron movement. This approximation is applicable to molecules rather than individual atoms, and its common simplification that "nuclei don't move" is misleading. The breakdown of the BO approximation often occurs in scenarios involving crossing electronic states, particularly at conical intersections, where the neglect of nuclear motion variations in electronic wave functions becomes critical.

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We can consider atomic nucleus fixed, so we suppose it doesn't move. But for what elements is Born–Oppenheimer approximation the least accurate (the nucleus moves a "a lot")?
I would say that for the elements with the lowest atomic numbers, because these elements have their nuclei the lightest and so they can move more and their movement influence electrons more than in some heavier elements, whose nuclei move less. Am I right or not?
 
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I don't understand the question. The BO approximation applies to molecules, not individual atoms, so I don't understand the consideration of elements.

Also, saying that the BO approximation is "nuclei don't move" is an oversimplification (although it is used often in elementary introductions to the subject). It is more that variations of electronic wave functions with respect to nuclear motion are neglected. In many molecules one will find, for example, crossing electronic states where the BO approximation breaks down. See for instance conical intersection.
 
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