Conceptual density functional theory for interacting species.

The perspective presented in this work shows explicitly that the first order perturbation based on the changes that occur in the chemical potential and the hardness of each of the interacting species, due to the change in their external potential that arises when they are in the presence of each other, can be translated into the language of conceptual density functional theory (CDFT). Through this approach, one is led to important insights about the relevant role played by the chemical potential, the hardness, the Fukui function and the dual descriptor to understand the chemical reactivity at the global level by means of the first two, and of the different sites within a molecule by means of the last two. In particular, it is shown that the covalent contribution to the interaction energy can be expressed in terms of these four indicators, and because of the close relation between them and their corresponding approximations in terms of the frontier orbitals, the present work highlights the fundamental role of frontier orbital theory in the study of chemical reactivity. The analysis presented of the "|Δμ| big is good" rule reinforces the validation of this theory from first principles, as Parr and Yang had anticipated in the work that gave rise to the Fukui function. Additionally, the simplified approach developed to study specific cases, together with the systems considered, shows that the perturbed descriptors provide relevant insights into the way in which the reactants approach each other and about the possible dominant effects present in a reaction.