We use the "isoconfigurational ensemble" [Phys. Rev. Lett. 93, 135701 (2004)] to analyze both dynamical and structural properties in simulations of a glass forming molecular liquid. We show that spatially correlated clusters of low potential energy molecules are observable on the time scale of structural relaxation, despite the absence of spatial correlations of potential energy in the instantaneous structure of the system. We find that these structural heterogeneities correlate with dynamical heterogeneities in the form of clusters of low molecular mobility.PACS numbers: 64.70. Pf,05.60.Cd,61.43.Fs,81.05.Kf Over the last decade, the identification and study of dynamic heterogeneity (DH), especially in computer simulations, has added an important new dimension to our understanding of complex relaxation in glass forming liquids [1,2]. DH refers to the emergence and growth of spatially correlated domains of mobile and immobile molecules as temperature T approaches the glass transition temperature T g . A question that dominates research on DH concerns its connection to the structure of the liquid: What local configurational properties influence whether a given molecule is mobile or immobile?Recent work by Harrowell and Fynewever [3] has shown conclusively that a structuredynamics connection must exist at the molecular level.To do so, they use an "isoconfigurational (IC) ensemble" [3, 4, 5], a set of microcanonical molecular dynamics (MD) trajectories in which each run starts from the same initial equilibrium configuration, but with molecular momenta chosen randomly accordingly to a MaxwellBoltzmann distribution. The result is a set of trajectories lying on the same energy surface, and evolving away from their common initial point in configuration space. They then define and evaluate the "dynamic propensity": the average, in the IC ensemble, of the squared displacement of a molecule at a time comparable to the structural relaxation time. They show that DH is observed in this approach, in the form of increasing spatial correlations of the dynamic propensities in a glass forming liquid as T → T g . Since the influence of the initial momenta is averaged over, the observed spatial correlations must be configurational in origin.The strength of Ref.[3] is that it exposed the features of DH that are structural in origin, without needing to determine what structural properties are responsible. Other studies have worked towards explicitly identifying structural correlators to dynamics. A number of works over the past decades have identified relationships between average structural properties (especially free volume and measures of symmetry in the local molecular environment) and bulk dynamics; see e.g. Refs. [6,7,8,9]. More recently, several studies have sought a correlation at the microscopic level, e.g. between local free volume and local mobility, with more success in some systems [10,11] than in others [12]. A notable absence of correlation between the local volume and the local Debye-Waller factor has been report...
Quantum and classical simulations are carried out on ice Ih over a range of temperatures utilizing the TIP4P water model. The rigid-body centroid molecular dynamics method employed allows for the investigation of equilibrium and dynamical properties of the quantum system. The impact of quantization on the local structure, as measured by the radial and spatial distribution functions, as well as the energy is presented. The effects of quantization on the lattice vibrations, associated with the molecular translations and librations, are also reported. Comparison of quantum and classical simulation results indicates that shifts in the average potential energy are equivalent to rising the temperature about 80 K and are therefore non-negligible. The energy shifts due to quantization and the quantum mechanical uncertainties observed in ice are smaller than the values previously reported for liquid water. Additionally, we carry out a comparative study of melting in our classical and quantum simulations and show that there are significant differences between classical and quantum ice.
The results of classical and quantum simulations of liquid water over a wide range of temperatures are compared to probe the impact of quantization on the properties of liquid water. We show that, when treated quantum mechanically, water molecules have an enhanced probability of accessing nontetrahedral coordination in the local three-dimensional structure. We discuss how this enhanced probability, also called "effective tunneling", is related to the dynamics of the hydrogen-bond breaking and molecular diffusion in the liquid. We explore in detail how local molecular environments affect the manifestation of quantum effects and identify a previously unreported and apparently unique behavior of the quantum mechanical uncertainty of the water molecule as a function of temperature. The nonmonotonic behavior of the quantum mechanical uncertainty with temperature is shown to be due to the notable strength of the water-water interaction in the condensed phase and becomes further evidence of the importance of the water structure in the properties of this ubiquitous liquid.
Fats are elastoplastic materials with a defined yield stress and flow behavior and the plasticity of a fat is central to its functionality. This plasticity is given by a complex tribological interplay between a crystalline phase structured as crystalline nanoplatelets (CNPs) and nanoplatelet aggregates and the liquid oil phase. Oil can be trapped within microscopic pores within the fat crystal network by capillary action, but it is believed that a significant amount of oil can be trapped by adsorption onto crystalline surfaces. This, however, remains to be proven. Further, the structural basis for the solid-liquid interaction remains a mystery. In this work, we demonstrate that the triglyceride liquid structure plays a key role in oil binding and that this binding could potentially be modulated by judicious engineering of liquid triglyceride structure. The enhancement of oil binding is central to many current developments in this area since an improvement in the health characteristics of fat and fat-structured food products entails a reduction in the amount of crystalline triacylglycerols (TAGs) and a relative increase in the amount of liquid TAGs. Excessive amounts of unbound, free oil, will lead to losses in functionality of this important food component. Engineering fats for enhanced oil binding capacity is thus central to the design of more healthy food products. To begin to address this, we modelled the interaction of triacylglycerol oils, triolein (OOO), 1,2-olein elaidin (OOE) and 1,2-elaidin olein (EEO) with a model crystalline nanoplatelet composed of tristearin in an undefined polymorphic form. The surface of the CNP in contact with the oil was assumed to be planar. We considered pure OOO and mixtures of OOO + OOE and OOO + EEO with 80% OOO. The last two cases were taken as approximations to high oleic sunflower oil (HOSO). The intent was to investigate whether phase separation on a nanoscale took place. We defined an "oil binding capacity" parameter, B(Q,Q'), relating a state Q to a reference state Q'. We used atomic scale molecular dynamics in the NVT ensemble and computed averages over 1-5 ns. We found that the probability of the OOE phase separating into a layer on the surface of the CNP compared to being retained randomly in an OOO + OOE mix were approximately equal. However, we found that it was probable that the EEO component of an OOO + EEO mix would phase separate and coat the surface of the CNP. These results suggest a mechanism whereby many-component oils undergo phase separation on a nanoscale so as to create a transition oil region between the surface of the CNP and the bulk major oil component (OOO in the case considered here) so as to create the appropriate oil binding capacity for the use to which it is put.
In this paper we analyze the atomic-level structure of solid/liquid interfaces of Lennard-Jones fcc systems. The 001, 011, and 111 faces are examined during steady-state growth and melting of these crystals. The mechanisms of crystallization and melting are explored using averaged configurations generated during these steady-state runs, where subsequent tagging and labeling of particles at the interface provide many insights into the detailed atomic behavior at the freezing and melting interfaces. The interfaces are generally found to be rough and we observe the structure of freezing and melting interfaces to be very similar. Large structural fluctuations with solidlike and liquidlike characteristics are apparent in both the freezing and melting interfaces. The behavior at the interface observed under either growth or melting conditions reflects a competition between ordering and disordering processes. In addition, we observe atom hopping that imparts liquidlike characteristics to the solid side of the interfaces for all three crystal faces. Solid order is observed to extend as rough, three-dimensional protuberances through the interface, particularly for the 001 and 011 faces. We are also able to reconcile our different measures for the interfacial width and address the onset of asymmetry in the growth rates at high rates of crystal growth/melting.
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