Strong-interaction limit of density-functional theory

Electrons can have a given smooth density distribution rho(r), even if thei r Coulomb interaction is scaled to infinity. This strong-interaction limit of density-functional theory provides essential information on the correlat ion energy of real electron systems. The simple concept of strictly correla ted electrons (SCE) is analyzed here as a model for that limit. SCE is solv ed exactly for any one-dimensional (1D) N-electron density and, in particul ar, for any 3D spherical two-electron system, such as the helium atom. Both the SCE interaction energy and the SCE external potential, which are obtai ned here as density functionals, obey all the relations known for the corre sponding quantities in the unknown true strong-interaction limit. At large but finite interaction, the electrons are still strongly correlated, perfor ming zero-point oscillations about the SCE limit. [S1050-2947(99)01312-8].