Evaluating common QTAIM and NCI interpretations of the electron density concentration through IQA interaction energies and 1D cross-sections of the electron and deformation density distributions

Nine kinds of inter- and intramolecular interactions were investigated by exploring the topology of electron density in the interatomic regions using standard protocols of QTAIM, IQA and NCI techniques as well as in-house developed cross-sections of the electron and deformation density distributions. The first four methods provide the properties of the resultant density distribution in a molecular system whereas the later illustrates the process, inflow or outflow of density from fragments to the interatomic region of an interaction on its formation in a molecular system. We used (i) the QTAIM-defined atomic interaction line, AIL (presence or absence), (ii) IQA-defined interaction energy, E int A,B , and its components, classical V cl A,B and exchange–correlation V XC A,B term, (iii) NCI-defined isosurfaces to identify local regions of accumulated (λ2 < 0) or depleted (λ2 > 0) density relative to immediate environment, and (iv) deformation density for which Δρ(r) > 0 indicates an inflow or otherwise an outflow of density on the interaction formation to explore the nature of the interactions. We found (i) AILs for highly attractive and repulsive interactions, regardless whether an inflow (Δρ(r) > 0) or outflow of density into the interatomic region, (ii) no correlation between the signs of λ2 and E int A,B ; both, highly repulsive and attractive, interactions might have locally depleted density and vice versa, (iii) locally accumulated density (λ2 < 0) does not imply that this is the result of an inflow (Δρ(r) > 0) of density and this equally applies to attractive and repulsive interactions either with or without an AIL. Results obtained demonstrate that the molecular environment can change the character of an interaction radically, from (i) attractive to repulsive, (ii) λ2 < 0 to λ2 > 0, or (iii) Δρ(r) > 0 to Δρ(r) < 0; hence, none of the topological indices used here, either separately or combined, can be used to definitely predict the (de)stabilizing nature of an interaction except highly repulsive ones for which the absence of AIL, interatomic density depletion and outflow of density on interaction formation are observed.