Help with symplectic integrators

Question

I\'m trying to develop a physics simulation and I want to implement a fourth-order symplectic integration method. The problem is that I must be getting the math wrong, since my

Answer 1

As you asked for source code: From HERE you can download MATLAB and FORTRAN code for symplectic methods for Hamiltonian systems and symmetric methods for reversible problems. And a lot of other methods for dealing with diff equations too.

And in THIS paper you can find the description of the algorithms.

Edit

If you use Mathematica this may help too.