where c 2 k [δ v (r)] is the oscillator strength of the kth excited state and E 0 -E k is minus the kth excitation energy. Therefore, eqn ( ) can be viewed as a minus the energy required to rear-range the electron configuration so that c 2 k electrons are promoted from the ground state to the kth state. Following the same line of thought, the plot of c 2 k versus (E 0 -E k ) can be considered as a polarization spectrum. The polarization density can be computed as