A constant-temperature non-equilibrium molecular-dynamics study was made of mass transport in an amorphous Lennard-Jones system prepared by rapidly cooling the liquid at constant atmospheric pressure. The phase diagram of the liquid-supercooled liquid glass was determined and the dependence of the glass stability upon the quenching rate was investigated. Mobility averages were calculated for when an external force was applied to the particles; thus bypassing the space-time limitations of equilibrium molecular dynamics which prevented the investigation of mass transport in glasses. The response of the system to the external perturbation was highly non-linear: at each temperature, T, a linear fit of the mobility, , versus the external force revealed the existence of a threshold perturbation above which the mobility became liquid-like (D = kT10-6cm2/s) the system still being amorphous. Below this threshold, the mobility was vanishingly small. The results showed that, for large perturbations, the mobility was temperature independent whereas the data were consistent with an Arrhenius behavior in the range of smaller perturbations.

Diffusion in the Lennard-Jones Glass Model Studied by Equilibrium and Non-Equilibrium Molecular Dynamics. Massobrio, C., Pontikis, V., Ciccotti, G.: Physical Review B, 1989, 39[4], 2640-53