Vibrational scaling of the heterogeneous dynamics detected by mutual information

Abstract

The correlations detected by the mutual information in the propensities of a molecular viscous liquid are studied by molecular-dynamics simulations. Dynamic heterogeneity is evidenced and two particle fractions with different mobility and relaxation identified. The two fractions exhibit scaling of their relaxation in terms of the rattling amplitude of the particle trapped in the cage of the first neighbours u 2 . The scaling master curve does not differ from the one found for bulk systems, thus confirming identical results previously reported in other systems with strong dynamic heterogeneity as thin molecular films. Excitation of planar and globular structures at short and long times with respect to structural relaxation, respectively, is revealed. Some of the globular structures are different from the ones evidenced in atomic mixtures. States with equal u 2 are found to have identical time dependence of several quantities, referring to both bulk and the two fractions with heterogeneous dynamics, at least up to the structural relaxation time τα.

Mutual Information in Molecular and Macromolecular Systems

“…If these are not known, their estimation is a delicate (and sometimes uncertain) procedure leading to higher computational cost [13,15]. Building on previous studies carried out by Molecular Dynamics (MD) simulations, the present paper reports novel insight provided by MI-based approaches on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity [23,24] and its influence on secondary relaxations [25,26]. Details about the model systems and numerical methods are found elsewhere [23][24][25][26].…”

“…They considered the MI between pairs of particles displacements, obtained a dynamic length scale growing on approaching GT, and revealed two distinct fractions of particles, so-called early and late fractions. To test its robustness, we extended the MI approach of Reference [18] to the context of molecular liquids and investigated by MD simulations [23,24]. For readers' convenience, these previous results are briefly outlined in Section 2.1.…”

“…MD simulations of a model molecular liquid made of fully flexible trimers have been carried out with technical details given elsewhere [23,24]. Relaxation and heterogeneous transport are characterised by the intermediate scattering function (ISF) and the non-Gaussian parameter (NGP) [36], respectively.…”

“…NGP is a well-known DH metrics [1,3,29]. By suitably adjusting both the density ρ and the temperature T of the liquid, six states were prepared to be grouped in three pairs, labelled A, B, and C, with coinciding ISFs (and then τ α ) and NGP; see Figure 2 and References [23,24] for details. Figure 2(left) shows that ISF has a characteristic plateau region at intermediate times.…”

“…We now show how MI can be employed to characterise DH. Additional details are given in References [23,24]. We consider the MI between the displacements of two generic particles i and j…”