All-electronab initioself-consistent-field study of electron transfer in scanning tunneling microscopy at large and small tip-sample separations: Supermolecule approach

Electron transfer expressed in the context of molecular-orbital theory is used as a model for scanning tunneling microscopy. We calculate the electronic coupling matrix element ${\mathit{T}}_{\mathit{a}\mathit{b}}$, ubiquitous in theories of electron transfer, by means of ab initio self-consistent-field wave functions for a supermolecule made up of a ‘‘sample’’ molecule and a ‘‘tip’’ metal atom. We find that ${\mathit{T}}_{\mathit{a}\mathit{b}}$ varies with the lateral position of the tip, with the tip-sample distance, and with the applied bias voltage. The features of the ${\mathit{T}}_{\mathit{a}\mathit{b}}$ curves are analyzed in terms of molecular orbitals.