Hello, using POP force inside of MPM solver gives me an error:
Increasing the substeps and decreasing CLF even to values like 0.1 dont change anything.

Is there a better way to add force?

Could you share your setup, hip file?
Personally I have used POP force with MPM many times and never had any errors like this.

It's hard to replicate that issue, and I can’t share the file because of NDA...but I figured it's because the particles cross more than one voxel size per substep. I hard-limited the velocity change, and it fixed the problem without changing the look at all.