These are relatively weak close-range interactions between atoms. Imagine two atoms sitting close together. At a given instant more of the electrons on one atom may be on one side, and this exposes the positive charge on the nucleus. This positive charge attracts the electrons of the adjacent atom thus exposing its nuclear charge, which would attract the electrons of another atom and so on. The next instant the electrons will have moved so we have a situation of fluctuating attractions between atoms. These forces are important in the close-packed interiors of proteins and membranes and in the specific binding of a ligand to its binding site.
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