Hence, a single NCMC move yields to one value of W CnAm and we performed 3000 NCMC moves to estimate W CnAm and obtain the chemical potential ∆µ CnAm during a MC simulationn by using our modified Widom technique. We present typical execution time of a NCMC move compared to standard moves implemented in RASPA in the Supplementary Information (Table ). A NCMC move is computationally expensive as its execution time is seven order of magnitude higher than those for translation and rotational moves, at the benefit of a proper equilibration of the system during the alchemical transformation. The calibration process of the chemical potential is performed for different electrolyte over a wide range of concentration (Table ). The Figure shows the evolution of the alchemical work and chemical potential for various electrolytes as a function of the electrolyte concentration. We could not explore KI electrolyte for concentration greater than 10 M (Figure -B, dark blue) because the largest anions I -(σ I -= 5.167 Å) could not be inserted in the box with other species. For all electrolyte, the alchemical work and the free energy are the highest (i.e., more negative) after insertion of a single electrolyte unit (c = 0.37 M), which is consistent with similar calculations performed on NaCl electrolyte. Upon increase of electrolyte units initially present in solution, the alchemical work required to transform water into ions decreases in absolute value as well as the chemical potential. Previous simulations of potassium halide aqueous solutions showed that the hydration number (i.e. the number of water molecules in the first hydration shell) of cations and anions drops while the number of ion pair increases slightly. This trend is consistent with the radial distribution functions (Figures S4 to S15