ON THE TOPOLOGY OF THE ELECTRON CHARGE-DENSITY AT THE BOND CRITICAL-POINT OF THE ELECTRON-PAIR BOND

Using the classical valence bond description of the electron-pair bond , as a resonance between a covalent structure and two ionic structures , we study the change in the topology of the charge density at the bon d critical point. In the first part of this paper, the density of the H-H and Li-H bonds is analyzed in terms of three types of contribution s: rho(A-B) = rho(cov) + rho(res) + rho(ion), the first contribution i s due to the covalent structure, the second to the resonance between c ovalent and ionic structures, and the last one comes from the ionic st ructures. From this analysis, we conclude that when the bond is descri bed as a covalent and one ionic structure, as in Li-H, the increase in the ionicity of the bond also corresponds with an increase in the clo sed-shell character of the electron density. However, in the case of t he H-H bond, where the two ionic structures are equally important, the increment in the shared type interaction is due to the resonance betw een covalent and ionic structures. In the second part of this paper, w e report an analysis of the classical valence bond description and the topological properties of the electron charge density calculated from ab initio GVB calculations for 15 different diatomic molecules at the equilibrium geometry and their dependence with the internuclear dista nce for H-2, LiH, F-2, Cl-2, Li-2, and Na-2 molecules. This analysis r eveals the importance of the overlap between the hybrid orbitals in a Heitler-London type wave function in determining the topological prope rties at the bond-critical point for covalent bonding. For Li-2 we hav e found that at the equilibrium distance, the topology of rho shows a maximum located at the middle of its bond, while for Cl-2 a similar ma ximum is found at shorter internuclear distances.