  In this paper, we study the exact dynamics of general open systems
interacting with its environment through particle exchanges. The paper includes
two main results. First, by taking advantage of the propagating function in the
coherent state representation, we solve the exact master equation, whose
solution is expressed in terms of the Keldysh nonequilibrium Green functions.
Second, in the dynamical perspective, we provide a rigorous thermalization
process of open quantum systems.
