TOPOLOGY OF DENSITY DIFFERENCE AND FORCE ANALYSIS .1. HOMOPOLAR BOND FORMATION

The density difference Delta rho(r) of a molecule A-B is defined as th e difference of the density rho(r) of the molecule A-B and the density rho(A)(r) + rho(B)(r) put at the position of the atoms A and B. We in vestigate here the topological features of the density difference and define electron density flow (EDF) as representing the direction and t he amount of the electron density flow in the course of the nuclear di splacement processes. As such examples, we study H-2 molecule formatio n reaction and the interaction of two He atoms. By the topological ana lysis of Delta rho(r), and by using the Hellmann-Feynman force and its partition into the AD, EC, and EGC forces, the characteristic behavio rs of the Delta rho(r) map are clarified. In particular, the electron cloud preceding and incomplete following are represented by using the concept of the EDF. The natures of the covalent bond are clarified bas ed on the topological properties of the difference density Delta rho(r ) rather than that of the total density rho(r).