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Equations of state and solid stuctures for a two-dimensional system of hard cyclic pentamers are obtained by the constant-pressure Monte Carlo method with a varying shape for the periodic box. Fluid and two crystalline phases are detected in the system. At high densities, a rectangular phase, with two pentamers per unit cell and (almost) pentagonal patterns of the atomic density distribution around each lattice site is observed. At intermediate densities, there is a hexagonal phase with a simple triangular lattice and (almost) hexagonal patterns of the atomic density distribution around lattice sites. The two first order phase transitions between the phases are located by a numerical determination of the free energies of the phases.
Brańka et al. (Fri,) studied this question.