  We develop a hierarchical functional derivative method to investigate the
reduced dynamics of a quantum dissipative system within the framework of a
stochastic decoupling description. Keeping only the lowest order truncation of
the hierarchical functional derivatives, one can recover the second order
Nakajima-Zwanzig quantum master equation. Taking into account the higher-order
corrections, our method can be implemented as a highly efficient numerical
scheme for a general quantum dissipative system beyond the usual Markovian,
rotating-wave, and weak-coupling approximations.
