Comment on "Breakdown of Interference Rules in Azulene, a Nonalternant   Hydrocarbon"
  Quantum interference (QI) effects in molecular systems are a topic of emerging interest in electron transport studies of single molecule junctions. In a recent Letter, Xia et al. employed a graphical scheme introduced by my colleagues and myself that enables to distinguish between molecular topologies in conjugated pi systems which exhibit QI effects in their conductance from those who do not. They claimed that this scheme is not applicable for nonalternant hydrocarbons, in particular Azulenes (Az), whose transport properties were studied both theoretically and experimentally by connecting the electrodes at Az's 1,3-, 2,6-, 4,7-, and 5,7-positions, respectively. An apparent disagreement has been identified between the predictions of our scheme referred to as "atom- or bond-counting model" and theoretical simulations for featureless, wide band electrodes in the GW approximation as well as for the tight-binding (TB) model our method was originally derived from.   In the following I would like to clarify that this apparent disagreement vanishes if the graphical scheme is applied correctly, providing a different result for the 1,3 Az compound than claimed by Xia et al. It will further be argued that the graphical scheme is completely general for any molecular topology in conjugated pi systems regardless of whether they are alternant or non-alternant hydrocarbons as long as the basic assumptions of its derivation are fulfilled. 
Robert Stadler
cond-mat.mes-hall