Constrained Simulations to Count the Glasses That a Lennard Jones Fluid Samples

The glass transition is usually regarded as a kinetic phenomenon but it can be treated in a purely thermodynamic way when physical constraints, rather than lack of time, are applied to prevent a liquid from sampling its many glasses. Simulations of Lennard Jones fluids show that they can be made glassy at any density or temperature and that the glass transition is accompanied by a decrease in the compressibility and heat capacity as at the kinetic glass transition. The constraints allow the free energy of individual glasses to be measured and this provides a direct way to count the number of glasses that a fluid samples. The number of glasses that the Lennard Jones fluid samples and its variation with temperature and density is predicted semiquantitatively using a temperature-dependent effective-hard-spherediameter to relate the number of Lennard Jones glasses to the number of hard sphere glasses. This suggests that the energy landscape picture should be broadened to an energy-density landscape picture.