Graphic description of the calculation of the excess chemical potential for an aqueous solution of sodium chloride as obtained with the adaptive resolution method (J. Chem. Theory Comput. 14, 7, 3409-3417 (2018)). The atomistic representation of the system is in thermal and chemical equilibrium with a reservoir of ideal gas particles (left panel). The whole system is at a constant chemical potential via the external potential applied to the molecules instantaneously present in the hybrid region (right panel).