where e -βW (n+m)H 2 O→CnAm N CnAm is the Boltzmann factor of the work performed for transforming alchemically water molecules into an electrolyte unit (denoted alchemical work from now on), averaging in the canonical ensemble of the system containing N CnAm electrolyte units. This adapted version of the Widom technique is slightly different from the original one applied to GCMC simulations as it requires only the difference of potential energy before and after insertion of a particle. In our case, we employ NCMC moves to compute the alchemical work W (n+m)H 2 O→CnAm (denoted W CnAm for clarity) performed during the alchemical transformation: (n + m) water molecules were chosen randomly in the box and alchemically transformed into an electrolyte unit C n A m over an alchemical path divided in T = 200 segments, each one consisting of a NCMC step where the system is perturbated and then relaxed. For a given segment t, the alchemical path is decribed by two parameters: the state of the system r t and the values of the interpolated non-bonded parameters λ t of the molecules undergoing alchemical transformation. In the same manner as in the work of Ross and co-workers, the alchemical path is a linear interpolation of the nonbonded parameters of water and salt ions, applied to randomly chosen (n+m) water molecules to be transformed into ions: