atoms, and the latter provides a new procedure to analyze electron pair localization. Electron localization function (ELF) [@Silvi:1994] was proposed for the study of electron pairing. ELF utilizes the gradient vector field topology to partition the electron density map into topological basins. In fact, a general theory called quantum chemical topology (QCT) [@Popelier:2005] has been developed for the topological analysis of electron density functions. Apart form the above mentioned AIM and ELF, QCT also includes the electrostatic potential [@Leboeuf:1999], electron localizability indicator (ELI) [@Kohout:2004], localized orbital locator (LOL) [@Schmider:2000], the virial field [@Keith:1996], the magnetically induced current distribution [@Keith:1993], the total energy (catchment regions) [@Mezey:1981] and the intracule density [@Cioslowski:1999]. QCT has proved to be very effective in analyzing interactions between atoms in molecular systems, particularly the covalent interactions and chemical structure of small molecular systems. Many software packages have been developed for QCT analysis [@Biegler:2002; @Henkelman:2006].

The essential idea behind QCT is the scalar field topology analysis [@Beketayev:2011; @Gunther:2014], which includes several major components such as critical points (CPs) and their classification, zero-flux interface (interatomic interface), gradient vector field, etc. In fact, when vector field topology is applied to the gradient of a scalar function, it coincides with scalar 