Weird bug causing simulation divergence in certain cases

[Wenjin-Xue]

meep_divergence_summary.zip
Hi all,

There seems to be a weird bug that causes simulation divergence for specific simulation setups.

The basic problem is that, sometimes the simulation will diverge when I add a block geometric object with the same material as the background material in the background region. However, when this block is not added, the simulation converges. Let's call this block as "redundant" block since it doesn't change the intended geometry for simulation.

The difference between the two simulations with and without "redundant" block is quite puzzling, and this phenomenon exists for various MEEP versions: v1.18.0, v1.23.0, v1.26.0, 1.27.0, 1.29.0. I didn't test other versions but I assume they are the same.

The example python code and the simulation output showing different results with and without "redundant" blocks are included in the attachment. The example python code can replicate the result described above. It would be great if you can figure out the reason and fix it, thank you very much!

Best,
Wenjin