When you get the error "step size too small", you can try the following to resolve it:
1. Try using a "lumped" of full Jacobian instead of the fixed one, which is the default. The Jacobian approximation method is selected in the Solve Parameters dialog box, which you open by selecting Parameters... from the Solve menu. Often, using the full or "lumped" Jacobian helps to achieve convergence at the cost of some extra computations. In this case, the "lumped" Jacobian works; the full Jacobian fails due to a singularity.
2. Specify a suitable initial solution. A good guess of what the soultion will look like can be used as a starting point for the nonlinear solver. The initial value can also be entered in the Solve Parameters dialog box. If you don't specify an initial solution, zero is used as the default.
