A view of bond formation in terms of molecular charge distributions

Abstract

The process of bond formation as a function of internuclear separation for H2 and Li2 is interpreted in terms of the changes in the charge distributions and the forces which they exert on the nuclei. The charge distributions are calculated from extended Hartree–Fock wave functions which reduce to the Hartree–Fock atomic functions for infinite nuclear separation. The results for H2 indicate that at separations greater than 5 a.u. the net attractive force exerted on the approaching nuclei arises from a simultaneous inwards polarization of the atomic charge distributions. For separations less than 5 a.u. the nuclei are bound by the force exerted by the delocalized component of the charge distribution. The density distributions and forces for He2 over a range of internuclear separations are compared with those for H2 to contrast the formation of stable and unstable molecular species in terms of their respective charge distributions.The final section of the paper examines in detail the changes in the Hartree–Fock molecular charge distribution which arise from the inclusion of electron correlation in the wave function. The maximum error in the Hartree–Fock charge distribution for H2 is found to be in the region between the nuclei, where it overestimates the charge density by approximately 1%. The errors in the Hartree–Fock charge distribution for Li2 are found to be of the same order of magnitude as the uncertainty in the calculated density distribution itself.