The idea of the IPEA shift is based on Koopmans' theorem for the closed-shell case, for which the orbital energies of the inactive and secondary orbitals can be written as the negative energies of the ionization potential (IP) and electron affinity (EA), respectively. In analogy, we want the orbital energies of the active orbitals to be the negatives of IP and EA for excitation out of and into partially occupied orbitals, respectively. Ghigo et al. expressed the Fock matrix as a weighted average of the IP and EA: