Simulating molecular conductance using real-time density functional theory

We present real-time density functional calculations of finite-bias conductance in a polyacetylene molecular wire. Our approach is based on a novel, efficient method for numerically propagating the time-dependent Kohn-Sham equations in a Gaussian basis. Localized density constraints are used to create an appropriate chemical potential bias that, when released, causes charges to flow from one end of the molecule to the other, generating a current. Our numerical scheme is efficient enough that one is able to perform “brute force” conductance calculations by simply increasing the size of the electron reservoirs and propagating until a reasonable average current can be extracted. We demonstrate the feasibility of this approach on a simple polyacetylene wire. By varying the size of the finite leads and comparing to commonly used nonequilibrium Green’s function calculations, we show that reliable current-voltage curves can be obtained from a finite length of the molecular wire, even though the system never reaches a steady state. Our results indicate that it should be technically feasible to perform the same type of “brute force” simulations on molecular junctions, although it seems unlikely that a true steady state will ever be reached in these cases, due to the greater significance of current fluctuations at low transmittance.