  There is a tremendous interest in fabricating superconducting flux circuits
that are nonstoquastic---i.e., have positive off-diagonal matrix elements---in
their qubit representation, as these circuits are thought to be unsimulable by
classical approaches and thus could play a key role in the demonstration of
speedups in quantum annealing protocols. We show however that the efficient
simulation of these systems is possible by the direct simulation of the flux
circuits. Our approach not only obviates the reduction to a qubit
representation but also produces results that are more in the spirit of the
experimental setup. We discuss the implications of our work. Specifically we
argue that our results cast doubt on the conception that superconducting flux
circuits represent the correct avenue for universal adiabatic quantum
computers.
