van der Waals interaction

Even electrically neutral atoms feel a faint stickiness when they come close. Electrons are always jiggling, so at any instant an atom's electron cloud is a little lopsided, creating a fleeting tiny charge that nudges its neighbor's cloud to match. These brief, flickering attractions are van der Waals interactions — individually almost nothing, but added over many atoms in close contact they become a real grip.

The attractive part, called the London dispersion force, falls off very steeply with distance, so it only matters when surfaces are nearly touching. Get too close, however, and the same atoms repel hard as their electron clouds overlap. The balance between this short-range pull and the sharp push defines an atom's effective size and explains why a ligand and pocket must match shape almost like a hand in a glove.

Because each contact is weak, van der Waals interactions reward total buried surface area rather than any single point. They are the quiet workhorse behind shape complementarity: a flat, featureless ligand surface that packs perfectly against the protein gains affinity from countless small contacts, even without any obvious hydrogen bonds or charges.