A 50‐Year Historical Perspective of the Extended Koopmans' Theorem, Its Exactness, and Future Prospects

The early history of the extended Koopmans' theorem is reviewed. Two research groups independently developed the extended Koopmans' theorem to calculate ionization energies from density matrices. Initially it was implemented for full configuration interaction and multi‐configurational self‐consistent field wave functions. Later it was implemented using other optimization methods. The long history of studies of the exactness of the extended Koopmans' theorem for ionization is discussed, as well as limits of accuracy for electron affinities. Its utility in obtaining Dyson orbitals for application in studies of chemical reactivity, for which orbital models have been instrumental to our understanding of chemical reactions, is discussed.