Koopmans' theorem

The Koopmans' theorem is a named after the Dutch physicist T. C. Koopmans approximation.

In it is assumed that the position of the energy levels of an atom or molecule does not change in its ionization. This is the ionization energy of the highest occupied molecular orbital ( Highest Occupied Molecular Orbital HOMO ) is equal to the negative orbital energy:

The Koopmans' theorem is an approximation, since vanishes at precisely this electron by the removal of an electron, the coupling of all the electrons and thus all the orbital energies change a little.

The orbital energy can also be calculated, for example by the Hartree- Fock method.