  A well known, if little documented, problem in many molecular simulations of
aqueous ionic solutions at finite concentrations is that unrealistic
cation-cation associations are frequently found. One might suspect a defect in
the ion-ion interaction potentials, about which not much is known. However, we
show that this phenomenon can also be traced to the fact that, in the
pair-potential approximation, the cation-water potentials are too deep compared
with the other ones and we investigate this phenomenon in some detail. We then
attempt to draw some general conclusions.
