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EN
We explore a simple example of a chaotic thermostated harmonic-oscillator system which exhibits qualitatively different local Lyapunov exponents for simple scale-model constant-volume transformations of its coordinate q and momentum p: {q, p} ! {(Q/s), (sP)}. The time-dependent thermostat variable ζ(t) is unchanged by such scaling. The original (qpζ) motion and the scale-model (QPζ) version of the motion are physically identical. But both the local Gram-Schmidt Lyapunov exponents and the related local “covariant” exponents change with the change of scale. Thus this model furnishes a clearcut chaotic time-reversible example showing how and why both the local Lyapunov exponents and covariant exponents vary with the scale factor s.
EN
A symplectic and time-reversible molecular dynamics algorithm is presented for rigid molecules in the quaternion representation. The algorithm is developed on the basis of the Trotter factorisation scheme using a Hamiltonian formalism The structure is similar to that of the velocity Verlet algorithm. Subsequently we describe the coupling of the rigid bodies to a thermostat. The isothermal molecular dynamics is defined by introducing additional pseudo-friction coefficients, according to a generalised Nose-Hoover prescription.
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