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Lu Jeu Sham and Walter Kohn developed self-consistent equations that make density functional theory computationally useful. The Kohn-Sham equations introduce an auxiliary system of noninteracting electrons with the same ground-state electron density as the interacting system. An effective potential includes the external potential, electrostatic repulsion and an exchange-correlation functional. Solving the equations and updating the electron density iteratively gives a self-consistent state. The difficult many-electron effects remain in the exchange-correlation functional, so practical calculations still need an approximation to it.^[ref]