This Demonstration shows the negative energy trajectories of a point mass moving in the radially symmetric potential U(x,y,z)-(x2+y2+z2)-α/2. Due to the radial symmetry the point mass moves in a two-dimensional plane perpendicular to the angular momentum vector.
The case of

corresponds to the Coulomb potential, where negative energy trajectories are periodic. For

, all the negative energy trajectories go through the origin.