Convergence problem: 3D photonic crystal waveguide

I’m trying to calculate hex PhC waveguide - bandstructure and group velocity but cannot find the source of the convergence problem. Still, I could get a reasonable band structure

But I cannot get the group velocity because of convergence. I tried to change boundaries, mesh, but I have very limited computer power, so maybe it was not enough to see the difference. Could you help me to find the problem? Please, find my file here:
Thanks in advance, Irina

Hi @kulkova,

Diverging simulations can have several possible causes. You mentioned the boundaries and the mesh, another parameter you can try and change is the “dt stability factor”. This parameter can be found in the FDTD properties, in the “Mesh settings” tab:

The default value is 0.99. You can try and reduce it. The downside, it will make the simulation longer. You may have to try several value before finding one that gets the simulation stable.

I hope this will help!