  Destructive quantum interference in electron transport through molecules
provides an unconventional route for suppressing electric current. In this work
we introduce "interference vectors" for each interference and use them to
characterize the interference. An interference vector may be an orbital of the
bare molecule, in which case the interference is very sensitive to
perturbation. In contrast, an interference vector may be a combination of
multiple molecular orbitals, leading to more robust interference that is
likelier to be experimentally observable. Our characterization scheme
quantifies these two possibilities through the degree of rotation and also
assigns an order to each interference that describes the shape of the
Landauer-B\"uttiker transmission function around the interference. Several
examples are then presented, showcasing the generality of our theory and
characterization scheme, which is not limited to specific classes of molecules
or particular molecule-electrode coupling patterns.
