(2.3.5) Yang : An exact DFT calculation for the ground state of an N electron system provides directly the ground state total energy E v (N) and electron density. It also provides the ground state energies for the corresponding (N -1) and (N + 1) electron systems directly through the chemical potentials of the N electron system. The extension of a similar connection to the excited states of the corresponding (N -1) and (N + 1) electron systems has recently been made. . However, since the exact functional is not available in an explicit form, neither is the method for the associated chemical potential calculations. We now focus on the discussion on explicit functional forms that include most existing DFAs.