The energy of the highest energy level in the ground state of a molecule that contains electrons, sometimes used as a measure of nucleophilicity in QSAR
models.
Source:
PAC, 2016, 88, 239. 'Glossary of terms used in computational drug design, part II (IUPAC Recommendations 2015)' on page 245 (https://doi.org/10.1515/pac-2012-1204)