A review on the information content of the pair density as a tool for the description of the electronic properties in molecular systems

Recent advances on the use of the information content of the pair density to understand bonding and reactivity of molecular systems are presented. Two quantities are described: (1) the information content of the like-spin exchange-correlation hole, χ_XC, and (2) the information content of the Coulomb hole, χ_C, of the like-spin and the unlike-spin cases. χ_XC is capable of describing the electron structure of molecules on the basis of chemical grounds with astonishing accuracy and provides a clear differentiation of the localization degree of electrons, belonging to different molecular basins. Furthermore, χ_XC is used as a tool to get insights in the electronic reorganization occurring along a reaction coordinate. In contrast to χ_XC, χ_C is observed to concentrate in regions between atomic shells, from which information of nonclassical weak interactions can be obtained. Some relevant problems, in which the application of χ_XC in conjunction with χ_C would be of great value, are presented.