Ionic interactions: Comparative topological approach

The NonCovalent Interaction (NCI) index applicability is extended to include ionic interactions, thus confirming its ability to detect both electronic and electrostatic effects. The characteristics of the reduced density gradient vs. the density, s(ρ), and its isosurfaces are modeled thanks to the promolecular model, enabling to deeply understand the topological features of the function in ionic compounds. The analysis is extended to periodic systems in order to shed light into solid-state interactions. NCI reveals that anion–anion interactions contribute to crystal stability. These characteristics are compared to complementary functions, such as AIM and ELF, providing a quantitative framework for their comparison through the validity of the promolecular model. Shifts between nonhomeomorphic density and ELF topologies are shown to be related to the relative hardness of ions.