A Monte Carlo molecular simulation that is based on the grand canonical thermodynamic ensemble, an ensemble (e.g., a system of particles) in statistical mechanics that describes the possible states of the particles in equilibrium. The model system properties of chemical potential, volume, and temperature are held constant, but the number of particles in the system (e.g., interlayer water molecules) is allowed to vary as equilibrium is achieved. In this context, equilibrium implies both potential energy and system density (number of particles). Monte Carlo moves are accepted based on the energy change of the move according to a Boltzmann probability (so that some moves resulting in higher energy are accepted).
Cf., force field, molecular dynamics, Monte Carlo molecular simulation