Strong pressure-energy correlations in liquids as a configuration space property: Simulations of tem- perature down jumps and crystallization

Computer simulations recently revealed that several liquids exhibit strong correlations between virial and potential energy equilibrium fluctuations in the NVT ensemble [U. R. Pedersen et al., Phys. Rev. Lett. 100, 015701 (2008)]. In order to investigate whether these correlations are present also far from equilibrium constant-volume aging following a temperature down jump from equilibrium was simulated for two strongly correlating liquids, an asymmetric dumbbell model and Lewis-Wahnström OTP, as well as for SPC water that is not strongly correlating. For the two strongly correlating liquids virial and potential energy follow each other closely during the aging towards equilibrium. For SPC water, on the other hand, virial and potential energy vary with little correlation as the system ages towards equilibrium. Further proof that strong pressure-energy correlations express a configuration space property comes from monitoring pressure and energy during the crystallization (reported here for the first time) of supercooled Lewis-Wahnström OTP at constant temperature. Introduction. – The subject of thermal equilibrium fluctuations is old and well understood. For large systems equilibrium fluctuations are approximately Gaussian. For any system the “Gaussian components” of the fluctuations – their correlation functions – determine the system’s response to external fields. This is summarized in the noted fluctuationdissipation theorem known for half a century [1], and little new is expected to be learned from studying equilibrium fluctuations It was recently shown that several model liquids exhibit strong correlations between the thermal equilibrium fluctuations of the configurational parts of pressure and energy at constant volume [2–5]. Recall that the pressure p is a sum of the ideal gas term NkBT/V and a term reflecting the interactions, W/V , where W is the so-called virial [1]: