To assess the distance distribution of aqueous ion pairs in the AdSoS setup, an additional ion was inserted into the FG region of the single-ion systems. While the first ion was positionally constrained to the box center, the second ion was subject to a half-harmonic flat-bottom distance restraint to the box center with a force constant of 3 • 10 3 kJ mol -1 nm -2 and an offset distance of 1 nm, in order to prevent it from leaving the FG region. Three combinations of ion pairs were considered: (i) Na + for both ions; (ii) Na -for both ions; and (iii) Na -as the center ion with Na + as the restrained ion. Only the charges of the ions were modified, their Lennard-Jones parameters were kept constant. The simulation settings were the same as for the single-ion simulations.