Reconsideration of chemical indices in conceptual density functional theory

The chemical indices of conceptual density functional theory are examined from the view point of the fundamental precepts for the chemical indices, i.e., observability, universality and mathematical rigour, for the energy derivatives with respect to the number of electrons and external potential up to the third order. Consequently, it is proposed that the mathematically rigorous Iczkowski-Margrave electronegativity, which is electron affinity, is more suggestive than the Pauling electronegativity. The second-order energy derivative with respect to the number of electrons, which is exactly zero, is available as an explicit constraint on the universal functional. Furthermore, charge transferability and orbital reactivity indices, which are the second-order energy derivative with respect to the number of electrons and external potential, are newly classified into the chemical indices of conceptual density functional theory.