In previous work, we have validated our GCMC and GCNCMC methods by reproducing the mass density of TIP3P water boxes. For fragment-water mixtures it is more appropriate to measure the bulk concentration of each fragment in water. We selected solutions of 0.5 M acetone and 0.1 M pyrimidine for this test as they do not aggregate in water at these concentrations. The starting concentrations for these tests were equilibrated boxes of pure water containing no other species, and boxes containing solutions of 1 M acetone and 0.5 M pyrimidine. To perform these simulations, we set the GCNCMC parameters appropriate for the target concentrations (see Methods).