Pioneering approaches to speed the equilibration of water molecules between bulk solvent and the protein interior have interleaved grand canonical Monte Carlo (GCMC) water moves with standard MD steps, and recent work has made this technology available in the OpenMM simulation toolkit. A GCMC move starts with an attempt ("try") to insert/remove a water molecule to/from a random point in the simulated system. Each try is stochastically accepted or rejected with a probability based on its energy and the target value of the chemical potential of water in the system being simulated. Because the random locations at which water moves are tried and potentially accepted can be in the protein interior or in bulk solvent, such moves are able to equilibrate water between the protein interior and exteriors, as desired.