in eq. ( ) reminds us of another textbook concept in chemistry: chemical hardness. This concept was proposed by Pearson in 1963 as an integral part of his hard and soft acids and bases (HSAB) theory. Together with Parr, Pearson later gave a quantitative definition, which became part of the conceptual density functional theory (CDFT) framework. Within CDFT, chemical hardness is defined as the negative of the derivative of electronegativity with respect to electron population. Since then, HSAB theory has been proven a useful tool in several chemical applications. What is the physical interpretation of chemical hardness ? Table . Components of the chemical potential 𝜇, evaluated for atoms in the first three rows of the periodic table. Values of 𝜇 are calculated via eq. ( ). Shown for comparison is the Fermi energy 𝜀 @ , evaluated as the average of occupied and unoccupied orbital energies. In the HSAB theory and CDFT, the hardness of a system, be it an atom, a molecule or an ion, has a well-established inverse cube root relationship to polarizability. Figure demonstrates that the exact same proportionality holds true between atomic polarizability and the negative of the 8〈= *+, 〉 <〈-〉 term in eq. ( ). The correlation is remarkably strong across atoms and a selection of alkali and alkaline-earth cations and halide anions (r 2 = 0.99). The relationship shown in Figure motivates us to define chemical hardness (within the EQC framework) as: