First-principles prediction of salt solubility and activity in water
In plain words
How much table salt dissolves in water, and how strongly the dissolved ions interact, are basic measured numbers. Computer models of salt water still fail to reproduce them without being fitted to them.
Precise statement
For NaCl in water at 298 K and ambient pressure, compute from a molecular model not fitted to these data the solubility (measured about 6.15 mol/kg) and the mean ionic activity coefficient $\gamma_{+-}(m)$ from 0.1 mol/kg to saturation within 5 percent, including the chemical potential of the crystal. An answer is a model and free-energy method meeting these targets that transfers to LiCl, KCl and CaCl2 without refitting.
What would settle it
Free-energy calculations with ab initio-quality or many-body polarizable potentials compared with tabulated activity and solubility data.