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We extend HBLSL's (Horwitz, Ben Zion, Lewkowicz, Schiffer and Levitan) new Riemannian geometric criterion for chaotic motion to Hamiltonian systems of weak coupling of potential and momenta by defining the 'mean unstable ratio'. We discuss the Dicke model of an unstable Hamiltonian system in detail and show that our results are in good agreement with that of the computation of Lyapunov characteristic exponents.

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The precise integration method can give precise numerical result for linear invariant dynamical system, and can be used to solve stiff linear invariant dynamical system. In this paper, precise integration is compounded with Runge–Kutta method and a new effective integration method is presented for solving nonlinear stiff problems. The numerical examples are given to demonstrate the validity and effectiveness of the proposed method.