  We present the reaction-coordinate polaron-transform (RCPT) framework for
generating effective Hamiltonian models to treat nonequilibrium open quantum
systems at strong coupling with their surroundings. Our approach, which is
based on two exact transformations of the Hamiltonian followed by its
controlled truncation, ends with a new Hamiltonian with a weakened coupling to
the environment. This new effective Hamiltonian mirrors the initial one, except
that its parameters are dressed by the system-bath couplings. The power and
elegance of the RCPT approach lie in its generality and in its mathematical
simplicity, allowing for analytic work and thus profound understanding of the
impact of strong system-bath coupling effects on open quantum system phenomena.
Examples interrogated in this work include canonical models for quantum
thermalization, charge and energy transport at the nanoscale, performance
bounds of quantum thermodynamical machines such as absorption refrigerators and
thermoelectric generators, as well as the equilibrium and nonequilibrium
behavior of many-body dissipative spin chains.
