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On the decay of the pair correlation function and the line of vanishing excess isothermal compressibility in simple fluids

Research output: Contribution to journalArticle (Academic Journal)

Original languageEnglish
Article number014501
Number of pages12
JournalJournal of Chemical Physics
Issue number1
Early online date2 Jul 2019
DateAccepted/In press - 1 Jun 2019
DateE-pub ahead of print - 2 Jul 2019
DatePublished (current) - 7 Jul 2019


We revisit the competition between attractive and repulsive interparticle forces in simple fluids and how this governs and connects the macroscopic phase behavior and structural properties, as manifested in pair correlation functions. We focus on the asymptotic decay of the total correlation function h(r) which is, in turn, controlled by the form of the pair direct correlation function c(r). The decay of rh(r) to zero can be exponential (monotonic) if attraction dominates repulsion and exponentially damped oscillatory otherwise. The Fisher-Widom (FW) line separates the phase diagram into two regions characterized by the two different types of asymptotic decays. We show that there is a new and physically intuitive thermodynamic criterion which approximates well the actual FW line. This new criterion defines a line where the isothermal compressibility takes its ideal gas value χT=χTid. We test our hypothesis by considering four commonly used models for simple fluids. In all cases, the new criterion yields a line in the phase diagram that is close to the actual FW line for the thermodynamic state points that are most relevant. We also investigate (Widom) lines of maximal correlation length, emphasizing the importance of distinguishing between the true and Ornstein-Zernike correlation lengths.

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