First-Principles Approach to the Conductance of Covalently Bound Molecular Junctions

We develop a first-principles approach for accurate conductance calculations of covalently bound molecular junctions. Our approach extends the DFT+Σ method, an approximate GW-based self-energy correction scheme acting on a tractable molecular subspace (based on a gas-phase reference of the same dimension) that corrects level alignment in the junction relative to density functional theory (DFT). We introduce a new extended gas-phase reference system, consisting of the molecule and several lead atoms, whose frontier orbitals maximally project onto the conducting orbitals of the junction. With this choice of reference, our self-energy correction to the Kohn–Sham Hamiltonian takes into account mixing of the gas-phase reference orbitals upon the formation of the junction. We apply our generalized DFT+Σ approach to a series of alkane–chain junctions in which the molecules are covalently bound to the leads via carboxyl terminal groups. Our results lead to conductance values in quantitative agreement with experim...