Efficient calculation of crystal-solution coexistence lines for aqueous electrolytes

Electrolyte solutions have a widespread presence in biological, geological, and industrial systems. To advance our understanding of these solutions, we need to develop theoretical models that can efficiently predict their collective properties. In this work, we present a novel workflow for computing the phase diagrams of electrolyte systems described by classical force fields, using free-energy calculations from molecular dynamics simulations. We show that this approach is significantly more efficient than commonly employed direct coexistence (interfacial) simulation methods. In particular, we apply this "chemical potential route" to obtain the NaCl crystal-aqueous solution phase diagrams for both pure and hydrated crystals for two parameterizations of the Madrid scaled-charge force field. We show that the original model parameterization achieves state-of-the-art performance in predicting NaCl-water phase behavior at 1 bar within the temperature range of 250-350 K and predicts a stable hydrohalite (NaCl · 2H2O) crystal at temperatures below 250 K. Our approach enables potential future computational studies of hydrohalite nucleation.