The chemical formation process from the study of radial intracule densities by constructing the relaxation holes, Dh(u), resulting from the difference between the actual radial intracule density and the nonrelaxed one, which is obtained from atomic radial intracule densities and the pair density constructed from the overlap of the atomic densities has been studied. Our results show that the internal reorganization of electron pairs prior to bond formation and the covalent bond formation from electrons in separate atoms are completely recognizable processes from the shape of the relaxation hole, Dh(u). The magnitude of Dh(u), the shape of Dh(u) for all u < Req and the distance between the minimum and the maximum in Dh(u) provide further information about the nature of the chemical bond formed. A computational affordable approach to calculate the radial intracule density from approximate pair densities has been also suggested, paving the way to study electronpair distributions in larger systems

This research has been funded by the Spanish MINECO/FEDER Projects CTQ2014-52525-P (E.M.), CTQ2014-54306-P (M.S.), CTQ2015-67608-P (X.L.), CTQ2015-67660-P (J.M.U.) and EUIN2017-88605 (E.M.), the Basque Country Consolidated Group Project No. IT588-13, the Generalitat de Catalunya (Project 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, and the ICREA Academia 2014 prize (M.S.)). The FEDER grant UNGI10-4E-801 (European fund for Regional Development) has also supported this research. M.R.M. acknowledges the Spanish Ministry of Education, Culture and Sports for the doctoral grant FPU-2013/00176 and M.V.N. the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) for the doctoral grant BES-2015-072734. The authors acknowledge the computational resources and technical and human support provided by the DIPC and the SGI/IZO-SGIker UPV/EHU