Publication

Abstract : Charge‐constrained calculations make it possible to rigorously analyze electron flow and electronegativity equalization in the process of bond formation. Such an analysis is performed for the prototypical H2, HF, and LiH molecules. As the bonds are stretched, the dependence of the electronegativity difference on the extent of charge transfer undergoes a transition from approximate linearity to a steplike discontinuous character. With the help of the second‐order perturbation theory, the bond hardness is related to the matrix elements of the fragment‐electron‐count operator and is shown to increase exponentially with the bond length R at the dissociation limit. For polar bonds, the magnitude of the in situ electronegativity difference ΔχAB decreases quickly with R due to the decreasing polarization of the fragments. However, ΔχAB levels off for large distances, and most of the reduction in charge transfer that accompanies bond dissociation can be attributed to the dramatic increase in the bond hardness. The charge‐constrained calculations provide both the evidence and explanation for the energy derivative discontinuities that are observed in isolated atoms and molecules.