(432b) Coarse Molecular-Dynamics Analysis Of Structural Transitions In Condensed Matter

Accurate determination of the onset of structural transitions in complex physical systems is a problem of crucial importance in condensed matter and materials physics. As direct access to such physical responses is typically difficult to attain experimentally, computational techniques such as molecular dynamics (MD) have become powerful tools for probing the underlying atomic-scale dynamics and determining the transition onsets. One of the most attractive features offered by MD lies in its ability to ultimately relate atomistic dynamics to macroscopically observable physical behavior; however, computing the evolution of all of the atomic coordinates over coarse time scales poses a severe limitation to the method. The limitation is compounded when addressing problems with multiple stable states that are separated by high energy barriers. In recent years, novel methods, such as hyperdynamics, transition path sampling, metadynamics, and coarse molecular dynamics (CMD), have been proposed to address long-time dynamics issues directly through atomistic simulation. In CMD, coarse-grained information is estimated on-the-fly from many short and properly initialized independent replica MD simulations. This information can then be used to identify transition points in the physical behavior of the complex systems under consideration. The method is based on the description of the evolution of the probability density, P(ψ,t), approximated by the Fokker-Planck equation where ψ(t) is an appropriate coarse-grained observable that describes the state of the system. In previous studies,^1,2 it has been demonstrated that the CMD method can be applied to determining accurately the thermodynamic melting point of a crystalline silicon model and the critical pressure that marks the onset of a polymorphic structural transition in a crystal under hydrostatic loading.

In this presentation, we report results of CMD analyses of various cases of structural transitions in condensed matter placing emphasis on order-to-disorder and on polymorphic transitions. Specific examples include the melting of bulk metallic crystals and the bcc-to-hcp polymorphic phase transitions undergone by model crystalline materials under high pressure loads. In the melting case, we reconstruct the underlying effective free-energy landscapes over a temperature range and use them to establish a relationship between the melting transition and the loss of structural and elastic stability of the bulk crystal. In the polymorphic transformation application, we extend our previous studies done in the low-temperature limit2 and include effective free-energy landscapes obtained over a wider temperature range. The new results allow determining the limits of inherent stability of the corresponding phases (i.e., bcc and hcp); the outcome of the analysis is a phase diagram, which demarcates the stability boundaries of each phase. We demonstrate that the CMD approach is of general applicability and may be particularly helpful in analyzing various other important types of structural-transition onsets in condensed matter and materials science; selecting appropriate coarse-grained variables is the key ingredient to the successful implementation of this approach through monitoring and analyzing the corresponding, properly initiated coarse-variable evolution.

1. M. A. Amat, I. G. Kevrekidis, and D. Maroudas, Phys. Rev. B 74, 132201 (2006).

2. M. A. Amat, I. G. Kevrekidis, and D. Maroudas, Appl. Phys. Lett. 90, 171910 (2007).