(2.2.1) Perdew : Often we need to predict the ground-state total energy and electron density of a system of real interacting electrons in a scalar external potential (created, for example, by their attraction to nuclei). Correlated wave-function theory provides "the right answer for the right reason", but at a high computational price for systems of many electrons. Kohn-Sham DFT employs a simpler noninteracting or Coulomb-uncorrelated wave function, but includes a density functional for the exchange-correlation energy that is exact in principle but requires improvable approximations in practice. It often provides "almost the right answer for almost the right reason at almost the right price" for real atoms, molecules, and materials. The noninteracting kinetic energy and the electron density are found by the not-so-expensive self-consistent solution of effective one-electron Schrödinger equations. Indeed, the exchange-correlation energy and exchange-correlation potential "exactify" the Hartree approximation for the ground-state energy and density. The generalization from total density to spin density 66 provides more information and enhances the accuracy of the approximations.