Fix-rigid different group of grains

Hi all!

I have to create a rigid body made by two differents group of atoms. From the fix-rigid documentation I used:

group grains union gran1 gran2

fix body grains group 2 gran1 gran2

However I have this error

ERROR: One or zero atoms in rigid body.

That’s no possible because I verified the presence of the atoms. What could be the problem?

thank you in advance for your help.


there is not enough information here to make a serious assessment.
the most likely cause is what the error message says.

please provide a complete log of the failing your run and - as per usual requirement - please let us know which LAMMPS version you are using and what platform/settings you are running on.