Koopmans' theorem is a concept in quantum chemistry that relates to the calculation of electronic energies and ionization potentials of molecules. Named after the Dutch physicist Bernard Koopmans, the theorem provides an important framework for understanding the relationships between molecular orbitals and the energies associated with removing electrons from a system.
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In frozen-orbital Hartree-Fock theory, removing an electron from occupied orbital changes the energy by approximately . Orbital relaxation and correlation are omitted, so a measured ionization energy is not an exact orbital-energy measurement.