Alternative density functional theory for atoms and molecules

A reference state whose electronic density is not necessarily equal to the true ground-state density is used in theoretical methods for electronic correlation in atoms and molecules. Because the density functional theory of Kohn and Sham postulates a model system for which these two density functions are identical, this postulate appears to preclude use of this model state as a reference state when formulating many-body corrections to the theory. This conflict is resolved by expressing the ground-state energy as a functional of the electronic density of a reference state determined by maximizing its projection on the true wavefunction.