The
Highest-energy
Occupied
Molecular
Orbital
(
HOMO) (filled or partly filled) and
Lowest-energy
Unoccupied
Molecular
Orbital
(
LUMO) (completely or partly vacant) of a
molecular entity. Examination of the mixing of frontier molecular orbitals of reacting molecular entities
affords an approach to the interpretation of reaction behaviour; this constitutes
a simplified perturbation
molecular orbital theory of chemical behaviour.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1116