  We investigate the role of the thermodynamic (TD) force, as an essential and
sufficient technical ingredient for an efficient and accurate adaptive
resolution algorithm. Such a force applied in the coupling region of an
adaptive resolution Molecular Dynamics (MD) set-up, assures thermodynamic
equilibrium between atomistically resolved and coarse-grained regions, allowing
the proper exchange of molecules. We numerically prove that indeed for systems
as relevant as liquid water and 1,3-dimethylimidazolium chloride ionic liquid,
the combined action of the TD force and thermostat allows for computationally
efficient and numerically accurate simulations, beyond the current capabilities
of adaptive resolution set-ups, which employ switching functions in the
coupling region.
