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Diagram of the HOMO and LUMO of a molecule. Each circle represents an electron in an orbital; when light of a high enough frequency is absorbed by an electron in the HOMO, it jumps to the LUMO.
3D model of the highest occupied molecular orbital in CO2
3D model of the lowest unoccupied molecular orbital in CO2

In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively.


The energy difference between the HOMO and LUMO is termed the HOMO–LUMO gap. HOMO and LUMO are sometimes called frontier orbitals in frontier molecular orbital theory. The difference in energy between these two frontier orbitals can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution.[1]


The HOMO level is to organic semiconductors roughly what the valence band maximum is to inorganic semiconductors and quantum dots. The same analogy can be made between the LUMO level and the conduction band minimum.[2]

Organometallic chemistryEdit

In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur.


A SOMO is a singly occupied molecular orbital such as half-filled HOMO of a radical.[3] This abbreviation may also be extended to semi occupied molecular orbital.

See alsoEdit


  1. ^ Griffith, J.S. and L.E. Orgel. "Ligand Field Theory". Q. Rev. Chem. Soc. 1957, 11, 381-383
  2. ^ Bredas, J,-L. "Mind the gap!". Mater. Horiz. 2014,1, 17-19
  3. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "SOMO". doi:10.1351/goldbook.S05765

External linksEdit