Lowest-energy molecular orbital that is unoccupied in the ground state but occupied by one or more electrons if the molecular entity occurs in its first electronically excited state.
See also: molecular orbital theory, energy band theory
Source:
PAC, 2022, 94, 15. 'Glossary of terms relating to electronic, photonic and magnetic properties of polymers (IUPAC Recommendations 2021)' on page 37 (https://doi.org/10.1515/pac-2020-0501)