  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.
