Molecular-dynamics ensembles: Fluctuations and correlations near the phase transitions

Abstract

Computer simulations of liquid and solid systems very close to the melting-freezing transition zone have been performed for the microcanonical, canonical, and isothermal-isobaric molecular-dynamics ensembles. Temperature, pressure, and density fluctuations were studied over long evolution times, and graphical and analytical statistical-error methods were used to investigate correlations in the data. The Nosé-Hoover (NH) method combined with the Toxvaerd algorithm is proposed as a correct method of obtaining the true fluctuation and correlation of the thermodynamic variables in the system, because the temperature and/or pressure constraints in the NH method do not affect the dynamical evolution of the system, and because the fifth-order Toxvaerd algorithm gives very accurate behavior for the correlations, as has been shown in recent studies.