Electrons are light and quick; atomic nuclei, comparatively, are heavy and slow. The gap in mass and time scales means that electronic and nuclear dynamics can usually be considered separately, as Max Born and J. Robert Oppenheimer pointed out in a 1927 paper1 that has heavily influenced how chemical physicists think about atoms and molecules to this day: Electrons gracefully flit around ponderous nuclei and instantaneously follow them wherever they go.
The intuitive Born–Oppenheimer picture doesn’t always hold, but it generally does a pretty good job. And in situations where it does break down, it’s usually clear what’s going on. In some chemical reactions, for example, the nuclei are known to rearrange too rapidly for the electrons to keep up (see Physics Today, August 2021, page 14).
Now, however, Kerstin Krüger of Georg-August University in Göttingen, Germany; her PhD advisers, Oliver Bünermann and Alec Wodtke; and their...
