Why does electron sharing lead to covalent bonding? A variational analysis.

Ground state energy differences between related systems can be elucidated by a comparative variational analysis of the energy functional, in which the concepts of variational kinetic pressure and variational electrostatic potential pull are found useful. This approach is applied to the formation of the bond in the hydrogen molecule ion. A highly accurate wavefunction is shown to be the superposition of two quasiatomic orbitals, each of which consists to 94% of the respective atomic 1s orbital, the remaining 6% deformation being 73% spherical and 27% nonspherical in character. The spherical deformation can be recovered to 99.9% by scaling the 1s orbital. These results quantify the conceptual metamorphosis of the free-atom wavefunction into the molecular wavefunction by orbital sharing, orbital contraction, and orbital polarization. Starting with the 1s orbital on one atom as the initial trial function, the value of the energy functional of the molecule at the equilibrium distance is stepwise lowered along several sequences of wavefunction modifications, whose energies monotonically decrease to the ground state energy of H2+. The contributions of sharing, contraction and polarization to the overall lowering of the energy functional and their kinetic and potential components exhibit a consistent pattern that can be related to the wavefunction changes on the basis of physical reasoning, including the virial theorem. It is found that orbital sharing lowers the variational kinetic energy pressure and that this is the essential cause of covalent bonding in this molecule. Copyright (c) 2006 Wiley Periodicals, Inc.