Charge transfer through molecular junctions within Redfield theory: subtleties and pitfalls

Charge transfer through nanoscale junctions connecting metallic leads with quantum dots or single molecules is often described within an open system formulation in terms of Redfield theory. Under non-equilibrium conditions, the usually invoked rotating wave approximation is not justified which may lead to unphysical steady state solutions with e.g. negative populations. In this work we explore subtleties and constraints of the approach and thus clarify its applicability in numerical calculations. General findings are illustrated for an analytically solvable case of a molecule with two electronic states.