= Exchange-correlation functional
{title2=$E_{\rm xc}[n]=F[n]-T_s[n]-E_H[n]$}
The part of the exact density functional left after subtracting the auxiliary noninteracting kinetic energy and classical <Hartree electrostatic energy>. It includes fermionic exchange, correlation and the difference between interacting and noninteracting kinetic energies. Its functional derivative enters the <Kohn-Sham equations>. Practical approximations to this functional, rather than the existence of a density description itself, are a central source of error.
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