The starting points for these tests were equilibrated boxes of pure water containing no other species, and also boxes containing solutions of 1 M acetone and 0.5 M pyrimidine. We then alternate between GCNCMC moves of the ligand and water to control the concentration of the system in the grand canonical ensemble (µVT). For every 20 ps of MD, one ligand move and three water moves were performed. The switching times for the ligand and water moves were 50 (n pert = 499, n prop = 50) and 10 (n pert = 99, n prop = 50) picoseconds respectively. The volume of the GCMC region was the volume of the system: 62 nm 3 .