In other words, do the electron shell arrangments act more like a cage to confine the nucleus (much like a pebble in a beachball), or does the larger moving momentum of the nucleus cause deformations of the electron shell...?
Michael Prescott (firstname.lastname@example.org)
The assumption is called the "Born-Oppenheimer approximation". It makes calculating the energy of an atom or a molecule much simpler. You can ignore the kinetic energy of the nuclei and assume that the sum of all the internuclear repulsions is a constant. Then the nuclear and the electron motions can be treated as two separate (and easier) problems.
Are you looking for situations where the motions aren't separable? It can and does happen, whenever the electrons can't adjust instantaneously to movements of the nuclei. Here are a few references that might be helpful (they're far above the general chemistry level!)
Author: Fred Senese email@example.com
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/atoms/faq/print-bo-approximation.shtml