  We provide a simple set of rules for predicting interference effects in
off-resonant transport through single-molecule junctions. These effects fall in
two classes, showing respectively an odd or an even number of nodes in the
linear conductance within a given molecular charge state, and we demonstrate
how to decide the interference class directly from the contacting geometry. For
neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan
pairing theorem to show that the interference class is decided simply by
tunneling on and off the molecule from same, or different sublattices. More
generally, we investigate a range of smaller molecules by means of exact diag-
onalization combined with a perturbative treatment of the molecule-lead tunnel
coupling. While these results generally agree well with GW calculations, they
are shown to be at odds with simpler mean-field treatments. For molecules with
spin-degenerate ground states, we show that for most junctions, interference
causes no transmission nodes, but argue that it may lead to a non-standard
gate-dependence of the zero-bias Kondo resonance.
