Atoms cohere to form solids largely due to exchange and correlation. The volume is set by a balance between the expansive electronic kinetic energy and the compressive exchange-correlation energy. These effects are simply illustrated by the jellium model, in which the valence electrons neutralize a positive background charge that is rigidly uniform. But the formation of free atoms under extreme expansion is found only in the deformable-jellium model. Deformable jellium is condensed matter in miniature, displaying not only bulk cohesion with a realistic equation of state and surface effects, but also phonons and plasmons and their soft mode instabilities. By drawing an analogy with the motion of shoppers in a mall, we also discuss an intuitive picture of exchange and correlation (the tendency of electrons not to bump into other electrons or into themselves).

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