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. FUKUI, YONEZAWA, and SHINGU (1952); FLEMING (1976). See also SOMO, subjacent orbital.
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