maximum hardness

https://doi.org/10.1351/goldbook.MT07068
A chemical system at a given temperature will evolve to a configuration of maximum absolute hardness, η, provided that the potential due to the nuclei, plus any external potential and the @ET07025@, remain constant. In terms of @M03996@ theory, the highest value of η reflects the highest possible energy gap between the lowest unoccupied and highest occupied molecular orbitals; this value correlates with the stability of a system.
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1950 [Terms] [Paper]