On the choice of diameters in a polydisperse model glassformer: deterministic or stochastic?

Abstract

In particle-based computer simulations of polydisperse glassforming systems, the particle diameters $\sigma = \sigma_1, \dots, \sigma_N$ of a system with $N$ particles are chosen with the intention to approximate a desired distribution density $f$ with the corresponding histogram. One method to accomplish this is to draw each diameter randomly from the density $f$. We refer to this stochastic scheme as model $\mathcal{S}$. Alternatively, one can apply a deterministic method, assigning an appropriate set of $N$ values to the diameters. We refer to this method as model $\mathcal{D}$. We show that especially for the glassy dynamics at low temperatures it matters whether one chooses model $\mathcal{S}$ or model $\mathcal{D}$. Using molecular dynamics computer simulation, we investigate a three-dimensional polydisperse non-additive soft-sphere system with $f(s) \sim s^{-3}$. The Swap Monte Carlo method is employed to obtain equilibrated samples at very low temperatures. We show that for model $\mathcal{S}$ the sample-to-sample fluctuations due to the quenched disorder imposed by the diameters $\sigma$ can be explained by an effective packing fraction. Dynamic susceptibilities in model $\mathcal{S}$ can be split into two terms: One that is of thermal nature and can be identified with the susceptibility of model $\mathcal{D}$, and another one originating from the disorder in $\sigma$. At low temperatures the latter contribution is the dominating term in the dynamic susceptibility.