Largest Lyapunov exponent in molecular systems. II: Quaternion coordinates and application to methane clusters.
We present a numerical procedure for extracting Lyapunov characteristic exponents from classical molecular-dynamics simulations of molecular systems. The theoretical frame chosen to describe the orientational degrees of freedom is the quaternions scheme. We apply the method to small methane clusters. Two different model potentials are used to investigate the role of internal molecular motion on the nonlinear dynamics, and several parameters are calculated to study the thermodynamics and chaotic dynamics of these clusters. Evidence is found for a solidlike to plasticlike phase transition occurring with the release of the orientational degrees of freedom, at low temperatures below the melting point. The largest Lyapunov exponent increases significantly during this transition, but it exhibits no particular variation during melting.
Calvo
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Laboratoire Collisions, Agrégats, Réactivité, CNRS UMR 5589, Institut de Recherche sur les Systèmes Atomiques et Moléculaires Complexes, Université Paul Sabatier, 118 Route de Narbonne, F31062 Toulouse Cedex, France. fcalvo@cea.fr