Density of States Simulations of Various Glass Formers

Abstract

The behavior of glassy systems in bulk and especially in confined geometries has received considerable attention over the last decades because of the technological importance and inherent complexity of the systems near or below the transition temperature. Confined glasses have been studied using different theoretical and experimental techniques which helped shape our understanding; but still huge gaps remain. In this work we are using the Wang–Landau Monte Carlo approach to study different model glasses. General Monte Carlo fails to sample all relevant regions of phase space; the application of this method gives us the opportunity to directly estimate the density of states and consequently any other thermodynamic properties. We can calculate properties in different ensembles using the same simulation runs. This random walk algorithm is designed to visit all energy states with equal probability to produce a flat histogram. We can estimate the density of states on the fly whenever any energy state is visited. We perform multiple simulations in overlapping energy regions and finally join them after proper scaling to obtain the overall density of states; the global density of states of the glass former is then known to within a constant. We apply this technique to a model binary Lennard Jones glass which is a well tested model, as well as for the first time to a realistic glass forming system, the small organic glass former Ortho–terphenyl (OTP). For OTP we start from a united atom model and derive systematically a coarse–grained representation by replacing each phenyl ring with one interaction site. We apply the Iterative Boltzmann Inversion for this purpose. This method relies on the structure of the atomistic model, mainly the radial distribution function (RDF). One needs to Boltzmann invert the atomistic RDF to obtain an initial guess for the non–bonded potential. Then using this potential for the preliminary coarse grained run gives a first set of RDFs to compare with the atomistic target RDFs. We then iterate this procedure until the structures coincide. After calibration of the mesoscale system against the atomistic model we estimate the density of states for this small organic glass former. We find that the properties of the bulk Lennard Jones model show very good agreement with literature values. The atomistic and coarse grained representations of Ortho–terphenyl in the bulk are in good agreement with experiments. Freestanding films show a lower glass transition than the bulk value for both models.