The most important principle of conceptual DFT relies on the assumption that any chemical process can be described by the response of the molecules to changes in the number of electrons N Z with regard to the presence of a constant nuclear potential ν(r) . The corresponding perturbation of the electronic ground-state energy E of a molecule upon reaction can be written as a truncated series up to second or higher order . The resulting reaction energy for a generic half reaction X+Y→ XY without back reaction and with reactants X and Y and product XY can be written as