  A general method of quantum-to-classical reduction of quantum dynamics is
described. The key aspect of our method is the similarity transformation of the
Liouvillian, which provides a new perspective. In conventional studies of
quantum energy transport, the rotating wave approximation has been frequently
regarded as an inappropriate approach because it causes the energy flow through
the system to vanish. Our formulation elucidates as to why this unphysical
result occurs and provides a solution for the problem. That is, not only the
density matrix but also the physical quantity is to be transformed. Moreover,
we show that quantum dynamics can be "exactly" replaced with classical
equations for the calculation of the transport efficiency.
