HOMO and LUMO
In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for "highest occupied molecular orbital" and "lowest unoccupied molecular orbital", respectively. HOMO–LUMO gap. The energy difference between the HOMO and LUMO is termed the "HOMO–LUMO gap". HOMO and LUMO are sometimes collectively called the "frontier orbitals", such as in the 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. Semiconductors. The HOMO level is to organic  semiconductors roughly what the maximum valence band is to inorganic semiconductors and quantum dots. The same analogy can be made between the LUMO level and the conduction band minimum. Organometallic chemistry. In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur. SOMO. A "SOMO" is a "singly occupied molecular orbital" such as half-filled HOMO of a radical. This abbreviation may also be extended to "semi occupied molecular orbital". Subadjacent orbitals: NHOMO and SLUMO. 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. They are named "NHOMO" for "next-to-highest occupied molecular orbital" and "SLUMO" for "second lowest unoccupied molecular orbital". These are also commonly referred to as HOMO-1 and LUMO+1, respectively.