Electron sharing indexes (ESI) have been applied to numerous bonding situations to provide an insight into the nature of the molecular electronic structures. Some of the most popular ESI given in the literature, namely, the delocalization index (DI), defined in the context of the quantum theory of atoms in molecules (QTAIM), and the Fuzzy-Atom bond order (FBO), are here calculated at a correlated level for a wide set of molecules. Both approaches are based on the same quantity, the exchange–correlation density, to recover the electron sharing extent, and their differences lie in the definition of an atom in a molecule. In addition, while FBO atomic regions enable accurate and fast integrations, QTAIM definition of an atom leads to atomic domains that occasionally make the integration over these ones rather cumbersome. Besides, when working with a many-body wavefunction one can decide whether to calculate the ESI from first-order density matrices, or from second-order ones. The former way is usually preferred, since it avoids the calculation of the second-order density matrix, which is difficult to handle. Results from both definitions are discussed. Although these indexes are quite similar in their definition and give similar descriptions, when analyzed in greater detail, they reproduce different features of the bonding. In this manuscript DI is shown to explain certain bonding situations that FBO fails to cope with. Finally, these indexes are applied to the description of the aromaticity, through the aromatic fluctuation (FLU) and the para-DI (PDI) indexes. FLU and PDI indexes have been successfully applied using the DI measures, but other ESI based on other partitions such as Fuzzy-Atom can be used. The results provided in this manuscript for carbon skeleton molecules encourage the use of FBO for FLU and PDI indexes even at the correlated level.

A decomposition of the second-order reduced density matrix provides natural orbital based indices that provide a separation of dynamic and nondynamic correlation.

The anisotropic molecular polarizability of the nitrate anion and its decomposition into atomic contributions is studied using ab initio quantum chemistry and the Atoms in Molecules theory. Aqueous solvation of NO₃ ⁻ in interfacial environments is investigated by a Car–Parrinello molecular dynamics simulation of a cluster, and classical molecular dynamics of an extended slab system with bulk interfaces using a polarizable force field based on the Atoms in Molecules analysis. Both in aqueous clusters and in systems with extended interfaces the nitrate anion clearly prefers interfacial over bulk solvation. This is primarily due to its large value of molecular polarizability, the gas phase value of which is reduced by only 5–10% in the aqueous environment. For polarizable force field simulations of ionic solvation, we recommend to cast the NO₃ ⁻ polarizability into three equal contributions of roughly 1.3 ų placed on the oxygen atoms of the anion.