  The Longuet-Higgins-Berry's phase has remarkable consequences for charge
transport in molecular rings. For generic (conical) crossing, where the phase
is $\pi$, a vanishing cause can lead to a diverging response in the amount of
charge transport. Away from level crossings, when the phase is 0, a vanishing
cause leads to a vanishing response. The divergence of the response near
crossing is related to, but distinct from, the divergence that occurs in the
generalized susceptibility. We illustrate this behavior for quantum models of
molecular rings driven by a running wave of small amplitude at zero and finite
temperatures.
