The maximum hardness principle in the Gyftopoulos-Hatsopoulos three-level model for an atomic or molecular species and its positive and negative ions

The maximum hardness principle is examined within the Gyftopoulos-Hatsopoulos three-level model for an electronic system. As an atom or a molecule approaches equilibrium at some chemical potential μ and constant temperature T, its softness (inverse hardness) is shown to most often approach a minimum value.