Term: Koopmans' theorem
https://doi.org/10.1351/goldbook.K03411

Definition:
Directly relates experimental ionization potentials with energy levels of molecular orbitals. The theorem states that the ionization potential required to remove an electron from the orbital Ψ i is given by the negative value of the energy of the orbital, -εi, as calculated within the Hartree–Fock approximation. The theorem is not applied to localized molecular orbitals, which are not eigenfunctions of the effective hamiltonian.

Related Terms:
1) ionization (http://doi.org/10.1351/goldbook.I03183).
2) ionization potential (http://doi.org/10.1351/goldbook.I03208).
3) hartree (http://doi.org/10.1351/goldbook.H02747).

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1949 (https://doi.org/10.1351/pac199971101919)

Citation: 'Koopmans' theorem' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.K03411

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