Abstract
Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in computer simulations of the Lennard-Jones liquid and other simple liquids, but not for hydrogen-bonding liquids such as methanol and water. The correlations that are present also in the crystal and glass phases reflect an effective inverse power-law repulsive potential dominating fluctuations, even at zero and slightly negative pressure. In experimental data for supercritical argon, the correlations are found to be approximately 96%. Consequences for viscous liquid dynamics are discussed.
Highlights
Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in computer simulations of the Lennard-Jones liquid and other simple liquids, but not for hydrogen-bonding liquids such as methanol and water
For any macroscopic system thermal fluctuations are small and apparently insignificant. Their significance, was pointed out by Einstein, who showed that for any system in equilibrium with its surroundings, the specific heat is determined by the magnitude of the energy fluctuations. This result may be generalized, and it has long been well understood that linear-response quantities are determined by equilibrium fluctuations of suitable quantities [1,2,3]
Wpow t Wt with a correlation coefficient of 0.99. These results prove that the repulsive core of the LJ potential dominates fluctuations, even at zero and slightly negative pressure, and that at a given state point it is well described by an inverse power-law potential
Summary
For any macroscopic system thermal fluctuations are small and apparently insignificant Their significance, was pointed out by Einstein, who showed that for any system in equilibrium with its surroundings, the specific heat is determined by the magnitude of the energy fluctuations. We here report strong correlations between instantaneous pressure and energy equilibrium fluctuations in one of the most studied models in the history of computer simulation, the Lennard-Jones liquid, as well as for other van der Waals liquids. These findings have significant consequences, in particular, for the dynamics of highly viscous liquids. The data include slightly negative pressure conditions, as well as three instances of the crystallized liquid (lower left corner)
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