Dynamics and instabilities near the glass transition: From clusters to crystals

Molecular dynamics simulation has been used to explore the evolution, kinetics, and dynamics of a liquid-glass transition in clusters and bulk matter. We demonstrate a dynamical indicator that characterizes the onset of the glass transition in clusters and is consistent with other indicators of glass transitions in bulk systems. This criterion, based on changes in chaotic behavior as measured by the largest Liapunov exponent, reveals aspects of the microscopic processes associated with the phase change from liquid to glass, and provides a connection between the thermodynamic and dynamical behavior of systems and their multidimensional potential surfaces. © 1998 American Institute of Physics. @S0021-9606~98!51601-7# When a liquid cools rapidly enough to avoid crystalliza- tion,1 it becomes a thermodynamically metastable fluid called a ''supercooled'' or ''undercooled'' liquid. With fur- ther cooling, the supercooled liquid transforms to an amor- phous solid phase or a glassy state.2 The glassy state of mat- ter, like the liquid state, has no long-range order but may have short-range order; unlike liquids, glasses have no ca- pacity to flow. This investigation explores molecular phe- nomena underlying supercooling and glass formation and en- ables a detailed quantitative description of this process and of the glass transition in small finite systems, in a manner consistent with the bulk glass transition. One quantity of par- ticular interest is the temperature at which the amorphous solid becomes unstable with respect to the corresponding crystal.3 At this temperature, various measurable quantities, such as the heat capacity and viscosity, show discontinuities like those at the melting or freezing transition. 2 Calorimetric and diffraction experiments, useful as they are, cannot probe microscopic changes near the transition temperature. At present, this understanding has to come from theoretical modeling and computer simulation. While plausible qualita- tive interpretations of dynamics and thermodynamics near the transition temperature are not difficult to find, generating a testable, quantitative description remains a formidable task. Part of this task is finding a suitable indicator to specify the phases and phase transition temperature of the system. Tra-