Koopmans' theorem

== English == === Etymology === Named after Dutch-American mathematician Tjalling Koopmans (1910–1985), who published this result in 1934. === Proper noun === Koopmans' theorem (chemistry) A theory stating that, in closed-shell Hartree-Fock theory, the first ionization energy of a molecular system is equal to the negative of the orbital energy of the highest occupied molecular orbital (HOMO).