Domain Decomposition Error

GROMACS version: 2020.5
GROMACS modification: No

I am getting the following Domain Decomposition error in gmx_mpi mdrun.
Fatal error:
There is no domain decomposition for 4 ranks that is compatible with the given
box and a minimum cell size of 12.8243 nm

My system has ~80K atoms in a cube of edge 9.33 nm. DD always fails because:

  1. Gromacs finds that maximum bonded distance to be 9.32 nm (due to a 1-4 pair at that distance)
  2. So minimum DD grid size is 12.8 nm (with some scaling of maximum bonded distance).
    Point 1. is strange because, across the periodic boundary, the distance between the pair is only 0.25 nm!
    Please let me know if you have any suggestions about what might be causing the error.

So, it turns out that because my system couple to itself across the periodic boundary, I have to use periodic-molecules = yes in the MDP file.