ELECTRON FLOW AND ELECTRONEGATIVITY EQUALIZATION IN THE PROCESS OF BOND FORMATION

Charge-constrained calculations make it possible to rigorously analyze electron flow and electronegativity equalization in the process of bo nd formation. Such an analysis is performed for the prototypical H-2, HF, and LiH molecules. As the bonds are stretched, the dependence of t he electronegativity difference on the extent of charge transfer under goes a transition from approximate linearity to a steplike discontinuo us character. With the help of the second-order perturbation theory, t he bond hardness is related to the matrix elements of the fragment-ele ctron-count operator and is shown to increase exponentially with the b ond length R at the dissociation limit. For polar bonds, the magnitude of the in situ electronegativity difference DELTA(chiAB) decreases qu ickly with R due to the decreasing polarization of the fragments. Howe ver, DELTA(chiAB) levels off for large distances, and most of the redu ction in charge transfer that accompanies bond dissociation can be att ributed to the dramatic increase in the bond hardness. The charge-cons trained calculations provide both the evidence and explanation for the energy derivative discontinuities that are observed in isolated atoms and molecules.