As described in Chap. 2, the static correlation (SC) plays an essential role in describing theoretically the formations and the dissociations of chemical bonds. On the basis of the novel density-functional theory (DFT) formulated in Chap. 3, we develop a static-correlation functional as a main subject in this chapter. In Sect. 4.1, we first discuss the characteristic property of the energy electron density \(n^e(\epsilon )\) on the energy coordinate \(\epsilon \) , which plays a central role in the construction of the SC functional. The advantage of using the density \(n^e(\epsilon )\) will be illustrated for the dissociating H \(_2\) molecule. At the dissociation limit of the molecule, the distribution \(n_1^e(\epsilon )\) for the spin-symmetry adapted density becomes identical to the distribution \(n_0^e(\epsilon )\) for the symmetry broken density, that is, \(n_1^e(\epsilon ) = n_0^e(\epsilon )\) . On the basis of this property, in Sect. 4.2, a simple exchange-correlation functional which includes the SC energy is developed using the energy distribution \(n_1^e(\epsilon )\) as an argument. In this chapter, the atomic units (a.u.) are used unless otherwise stated.