Electrical conductance of individual molecules

The dc electrical conductance of several di-substituted benzenes, as well as pyrazine and biphenyl, linking two metal electrodes is computed using the density functional formalism. Possible conduction mechanisms are discussed. Only pyrazine has a nearly half occupied resonance at the Fermi level, leading to a conductance of roughly 1 quantum unit (2e²/h). In all of the other systems, the Fermi level falls between the HOMO and LUMO levels, but the density of states there is never zero, because hybridization with the metal states broadens the molecular levels to produce a finite state density between levels. The conductances for these other molecules fall in the range 0.08--0.47 quantum units (for high-electron-density electrodes).