The Role of Attractive Interactions in Self-Diffusion

Abstract

Recently, an alternative approach to self-diffusion in atomic liquids was proposed by one of us Szamel, G. Chem. Phys. 1999, 110, 3009]. This approach is applicable where the concept of binary collisions breaks down and the self-diffusion coefficient is small. Predictions from this method are in quantitative agreement with molecular dynamics (MD) simulations, over a broad range of densities and temperatures, for an atomic liquid interacting with a repulsive r -12 potential. Here we extend this approach to include attractive interactions; we study a liquid interacting with the Lennard-Jones (LJ) potential. Theoretical predictions are compared to MD simulations results. To clarify the role of attractive interactions, we compare LJ results with those obtained with the repulsive part of the LJ potential. Conclusions about the role of the attractive forces in self-diffusion are discussed.

Critical test of the mode-coupling theory of the glass transition

“…The different values found for the two models in the high-temperature regime reflect the fact that attractive forces already play an important quantitative role in this regime, despite the very little changes observed in static quantities. This effect was reported before in the case of simple liquids [20][21][22]. The role of attractive forces at high temperatures is partially captured by our MCT calculations, since we find theoretically a 25 % change in E ∞ between the two models.…”

Instantaneous normal modes analysis of amorphous and supercooled silica

“…For example, in our earlier work on simple glasses, 12,13 we found that the imaginary frequency modes of such glasses were localized in space for very low temperatures, with extended modes only appearing above the glass transition temperature. To quantify the spatial extent of the normalized eigenvectors (e ␣ i , of each INM, where i runs over the N particles in the sample and ␣ labels the modes͒, we utilize a standard measure of localization, namely, the participation ratio, first introduced by Bell and Dean…”

“…12,13,21,22 The INMs are defined in analogy with the more familiar normal modes. For an N-particle system at a given temperature T one chooses a configuration ͑defined by a 3N-dimensional vector of atomic coordinates, R 0 ) from the trajectory at some time t 0 .…”

“…Although they cannot be directly interpreted on an individual basis as representing barrier crossing events, 23 they do give a qualitative indication of the types of motions in the system that lead to structural relaxation, which is especially useful in the study of glasses and supercooled liquids. 13 For a given temperature and density, the normalized INM density of states ͑DOS͒ is defined as…”

“…In this paper, we extend the analysis of Taraskin and Elliot to amorphous and supercooled silica at nonzero temperature by using instantaneous normal mode analysis, a technique that we have applied previously to simple monatomic, fragile glass formers, with some success. 12,13 …”