Electron correlation effects on the shape of the Kohn-Sham molecular orbital

Abstract

Even systems in which strong electron correlation effects are present, such as the large near-degeneracy correlation in a dissociating electron pair bond exemplified by stretched H2, are represented in the Kohn–Sham (KS) model of non-interacting electrons by a determinantal wavefunction built from the KS molecular orbitals. As a contribution to the discussion on the status and meaning of the KS orbitals we investigate, for the prototype system of H2 at large bond distance, and also for a one-dimensional molecular model, how the electron correlation effects show up in the shape of the KS σg orbital. KS orbitals φHL and φFCI obtained from the correlated Heitler-London and full configuration interaction wavefunctions are compared to the orbital φLCAO, the traditional linear combination of atomic orbitals (LCAO) form of the (approximate) Hartree-Fock orbital. Electron correlation manifests itself in an essentially non-LCAO structure of the KS orbitals φHL and φFCI around the bond midpoint, which shows up particularly clearly in the Laplacian of the KS orbital. There are corresponding features in the kinetic energy density ts of the KS system (a well around the bond midpoint) and in the one-electron KS potential vs (a peak). The KS features are lacking in the Hartree-Fock orbital, in a minimal LCAO approximation as well as in the exact one.