A unified approach to Hamiltonian systems, Poisson systems, gradient   systems, and systems with Lyapunov functions and/or first integrals
  Systems with a first integral (i.e., constant of motion) or a Lyapunov function can be written as ``linear-gradient systems'' $\dot x= L(x)\nabla V(x)$ for an appropriate matrix function $L$, with a generalization to several integrals or Lyapunov functions. The discrete-time analogue, $\Delta x/\Delta t = L \bar\nabla V$ where $\bar\nabla$ is a ``discrete gradient,'' preserves $V$ as an integral or Lyapunov function, respectively. 
Robert I McLachlan, GRW Quispel, and Nicolas Robidoux