Molecular dynamics study of binary soft-sphere mixtures: Jump motions of atoms in the glassy state

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Abstract
Three very long runs of constant temperature molecular dynamics (MD) simulations, extending over typically 2×105 time steps, have been carried out for a state just above the glass transition of equimolar soft‐sphere mixtures, with 500 atoms interacting through vαβ(r) =ε(σαβ /r)12, where σαβ =(σα +σβ )/2, and α,β (species indices)=1, 2. The ratio of diameters σ2/σ1 was chosen to be 1.2. Jump‐type atomic motions are clearly found to occur in the glassy states. A coordinated (strongly correlated) jump motion was observed, where a cluster of several atoms dynamically linked at nearest‐neighbor distances jump at successive close times. These simulations demonstrate that the characteristic time scale of structural relaxation becomes very long near the glass transition, and calculated mean square displacements exhibit complicated behaviors. Thermodynamic quantities, however, such as the mean pressure, energy, and pair distribution functions, appear to behave normaly, with no significant observable relaxations for increasing time, so that their averages are well defined.

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