Density functional theory and Bader's atoms-in-molecules theory share the same primary ingredient: the electron density, which is the fundamental physical observable in quantum chemistry. In this paper, we elaborate on the decomposition of the Kohn–Sham molecular energy in terms of Bader's partition, discussing how Pendás' Interacting Quantum Atoms framework could be adapted to a DFT context. Besides, another bridge between these two theories is built through a general formalism able to generate new local descriptors from any second-order density gradient expansion. These approaches are then applied to two classes of intramolecular bonds: between two electronegative atoms and intramolecular hydrogen bonds, illustrating how fruitful a dialog between both theories can be.

In this paper, we present a detailed energetic decomposition of intramolecular O⋯X interactions (X being O, S, or a halogen atom) based on the interacting quantum atoms approach of Pendás and co-workers. The nature of these interactions (repulsive or attractive, more or less electrostatic) is discussed in the framework of Bader's atoms in molecules theory, a particular emphasis being put on delocalization (measured by delocalization indexes and in terms of the source function) and on the exchange contributions. Notably, the concept of exchange channels introduced by Pendás and collaborators provides means of rationalizing and predicting the presence of bond critical points, enhancing the physical meaning of bond paths.

In this paper, we introduce new local descriptors in the framework of Conceptual Density Functional Theory. They can be considered as an extension of the dual descriptor [Morell et al., J. Phys. Chem. A, 2005, 109, 205]. These indices are particularly suited for the discrimination between electrophilic and nucleophilic sites inside a molecule. They are computed using the densities of the electronic excited states, giving a picture of the polarization of the electron density induced by the approach of a reactant. Links with the linear-response function are discussed, and the first examples of applications are given, highlighting how these new descriptors can be used in practice for reactivity studies. It has been found that this extension of the dual descriptor can handle tricky cases, such as nitrobenzene or isoquinoline, for which Frontier Molecular Orbital Theory fails.