Cumulene molecular wire conductance from first principles

We present first principles calculations of current-voltage characteristics (IVC) and conductance of $\text{Au}(111):{\text{S}}_{2}$-cumulene-${\text{S}}_{2}:\text{Au}(111)$ molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio nonequilibrium Green's function density-functional theory methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metalliclike ballistic transport behavior. The reason is the high density of states in broad lowest unoccupied molecular orbital levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region of $\ifmmode\pm\else\textpm\fi{}1\text{ }\text{V}$. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.