Modeling the charge dependence of total energy and its relevance to electrophilicity

Several models for the charge dependence of the energy of ground-state atoms, molecules, atoms in molecules, and groups in valence states are reviewed. Compelling reasons for promotion into Ruedenberg-type valence states are presented. A recently introduced electrophilicity index, viz., electronegativity squared divided by hardness, is shown to be a common feature of all of the models and is validated as a measure for the energy difference between the neutral gas-phase species and the one saturated with electrons.