Role of the exchange-correlation energy: Nature's glue

In the Kohn–Sham density functional theory of ground-state electronic structure, only the exchange–correlation energy Exc must be approximated. Although Exc is not typically a large component of the total energy, it is the principal ingredient of the glue that binds atoms together to form molecules and solids. To illustrate this fact, we present self-consistent results for atomization energies of molecules and for surface energies and work functions of jellium, calculated within the “Hartree” approximation, which neglects Exc. The Hartree world displays weak bonding between atoms, low or negative surface energies, and work functions that are close to zero. Other aspects of the Hartree world can be deduced from known size–effect relationships. The mechanism behind the glue role of exchange and correlation is the suppression of Hartree charge fluctuations.