We implemented the osmostat in RASPA so that electrolyte units can be added or removed during a simulation, in the same way as in the work of Ross and co-workers. By setting up the osmostat, a system is driven to chemical equilibrium with a saline reservoir at a given electrolyte concentration that required the calibration of ∆µ CnAm previously done in the first section. The maximal number of electrolyte units N max CnAm that can be present in the system is the floor function of N H 2 O /(n + m) where N H 2 O is the number of water molecules in the bulk system chosen as an initial configuration (150 water molecules). We briefly present a description of the osmostat implementation in RASPA.