Koopmans theorem

The Koopmans theorem is an approximation named after the Dutch physicist T. C. Koopmans .

In it it is assumed that the position of the energy levels of an atom or molecule does not change when it is ionized . Characterized is ionization energy  of the highest occupied molecular orbital ( Highest Occupied Molecular Orbital HOMO) equal to the negative orbital energy  : ${\displaystyle I\,\!}$ ${\displaystyle \varepsilon \,\!}$

${\displaystyle I=-\varepsilon .\,\!}$

The Koopmans theorem is an approximation, since by removing an electron  the coupling of all electrons to this electron disappears and so all orbital energies change a little bit. ${\displaystyle i\,\!}$

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

See also

• Photoelectron Spectroscopy (PES)
• Ultraviolet Photoelectron Spectroscopy (UPS)
• X-ray Photoelectron Spectroscopy (XPS)
• Density Functional Theory (Quantum Physics) (DFT)

literature

• T. Koopmans: About the assignment of wave functions and eigenvalues ​​to the individual electrons of an atom . Physica (Amsterdam) 1.104, 1934, doi: 10.1016 / S0031-8914 (34) 90011-2
• A. Szabo, NS Ostlund: Modern Quantum Chemistry , Dover Publications, Mineola 1996, ISBN 0-07-062739-8