Excess entropy and structural transitions in a two-dimensional square-shoulder fluid.

Metropolis Monte Carlo simulations on the square-shoulder fluid of Malescio and Pellicane are used to trace the temperature dependent excess entropy, the heat capacity, and configurational energy along several isochores, including those for which mechanically stable zero-temperature structures have been predicted. Thermodynamic signatures of structural phase transitions are identified along several isochores, in addition to the low-density triangular solid and stripe phase transitions identified earlier. The finite temperature phases illustrate the competition between cluster formation and stripe formation as competing mechanisms for generating minimum free energy configurations as a function of density, consistent with earlier results at zero temperature. We also critically examine the usefulness of a phase-ordering rule based on the residual multiparticle entropy (RMPE) in predicting the formation of this diverse set of ordered structures from a disordered fluid phase. For the majority of the isochores studied, the RMPE prediction and the thermodynamic evidence for a phase transition were consistent. However, this criterion fails along isochores that are in regions of coexistence. Thus, the zero-RMPE rule is only likely to be approximately predictive in systems with small phase coexistence regimes, e.g., in the case of liquid crystal forming systems.