  The simulation of open molecular systems requires explicit or implicit
reservoirs of energy and particles. Whereas full atomistic resolution is
desired in the region of interest, there is some freedom in the implementation
of the reservoirs. Here, we construct a combined, explicit reservoir by
interfacing the atomistic region with regions of point-like, non-interacting
particles (tracers) embedded in a thermodynamic mean field. The tracer
molecules acquire atomistic resolution upon entering the atomistic region and
equilibrate with this environment, while atomistic molecules become tracers
governed by an effective mean-field potential after crossing the atomistic
boundary. The approach is extensively tested on thermodynamic, structural, and
dynamic properties of liquid water. Conceptual and numerical advantages of the
procedure as well as new perspectives are highlighted and discussed.
