The atoms in molecules (AIM) and natural population (NP) analyses have been used for a number of points along the intrinsic reaction coordinate (IRC) of some backside identity S(N)2 reactions (X- + CH3X = XCH3 + X-; X = F-, Cl, Br). Several integrated properties over the atomic basins (charge, atomic energy, Hartree-Fock energy, and atomic volume) have been obtained in natural coordinates and the changes of them have been calculated along the IRC. In addition, results of natural population analysis (natural charges, occupation numbers, and donor-acceptor interactions) have also been considered along the IRC. (c) 2005 Elsevier B.V. All rights reserved.