  Recent development of theoretical method for dissipative quantum transport
have achieved notable progresses in the weak or strong electron-phonon coupling
regime. However, a generalized theory for dissipative quantum transport at
arbitrary parameter regime is not figured out until now. In this work, a
variational method for dissipative quantum transport at arbitrary
electron-phonon coupling regime is developed by employing variational polaron
theory. The optimal polaron transformation is determined by the optimization of
the Feynman-Bogoliubov upper bound of free energy. % which is variational in
nature. The free energy minimization ends up with an optimal mean-field
Hamiltonian and a minimal interaction Hamiltonian. Hence, second-order
perturbation can be applied to the transformed system, resulting an accurate
and efficient method for the treatment of dissipative quantum transport with
arbitrary electron-phonon coupling strength. Numerical benchmark calculation on
an single site model with coupling to one phonon mode is also presented.
