I've just encountered a strange problem: I can't evolve Minkowski with MoL's RK4 integrator. Small errors in K grow from floating-point round-off, and the simulation quickly goes bad. The error grows slowly (but measurably) in unigrid, but much faster with mesh refinement.
For example, with unigrid the lapse grows by about 10^-15 every ten time steps or so; with mesh refinement, the simulation dies after a few ten time steps.
However, things are fine with other time integrators, in particular with the generic RK1 and RK4 integrators.
I've looked at other things as well, e.g. the CFL factor, the McLachlan implementation, the gauge parameters, dissipation, but could not find anything else that made a difference.
Is there something bad about the RK4 integrator? Is it intrinsically unstable, more so than other RK schemes? Or did it break in some of the recent changes?
-erik