All these response functions and some others derived from the energy function E(N), of which the electrophilicity ω is the most eminent representative, were shown to bear chemical relevance. To give some examples: the chemical hardness η in eqn ( ) was identified with Pearson's hardness, while the Fukui function f (r) was recognized as a generalization of Fukui's frontier MO concept, its product with the total softness, the local softness s(r) = S f (r) being a local indicator for soft regions in a molecule.