where it is assumed that the electronegativity and chemical hardness of the reference state are χ R = η R = 0. The previous relation exactly corresponds to Eqn. ( ) and Eqn. ( ) such that ∆E ZR = ∆E ZS = -ω Z . As a side remark, one has to notice that the back-reaction from the gas phase to the solid state results in E ZZ = 0. If we assume that ∆E ZR ∝ -∆H V , one can immediately see from Eqn. ( ) that