Current–voltage curves for molecular junctions: the issue of the basis set for the metal contacts

摘要

We present current–voltage (I–V) curves for a chain of six Au atoms and for benzene-1,4-dithiol between two Au (111) surfaces computed using a self-consistent, non-equilibrium, Green’s Functions approach in conjunction with a density functional theory (DFT) description of the extended molecule. The solutions are expanded in Gaussian basis sets. In an attempt to improve the description of the molecule-surface bond, we consider one- and two-layer clusters of metal atoms in the extended molecule. To avoid non-physical charging of the bulk metal clusters we add a shift to the diagonal elements of the Fock Matrix of the metal atoms in the DFT treatment; this can be viewed as correcting the self-energies or shifting the Fermi level. The Au6 cluster with the full minimal basis set represents a compromise between accuracy and cost. The Au31(12/19) two-layer cluster results are of about the same quality as the one-layer Au21 cluster, but allows one to study more binding sites. Using a double zeta basis set for three of the top-layer Au atoms of the Au31(12/19) cluster yields result similar to the all minimal basis set, while using all double zeta atoms yields inferior results.