The hard hexagon model occurs within the framework of the grand canonical ensemble, where the total number of particles (the "hexagons") is allowed to vary naturally, and is fixed by a chemical potential. In the hard hexagon model, all valid states have zero energy, and so the only important thermodynamic control variable is the ratio of chemical potential to temperature μ /(kT). The exponential of this ratio, z = exp(μ /(kT)) is called the activity and larger values correspond roughly to denser configurations.