HOMO and LUMO

Its size can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution.

The same analogy can be made between the LUMO level and the conduction band minimum.

[3] 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.

If existent, the molecular orbitals at one energy level below the HOMO and one energy level above the LUMO are also found to play a role in frontier molecular orbital theory.

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 CO 2
3D model of the lowest unoccupied molecular orbital in CO 2