Follow-up Comment #7, bug #24131 (project relax):
To add on my earlier comment: 
When encountering a spin without data, I get the above error message about
missing interatomic interaction. 
If I define interactions with:
  interatom.define(spin_id1='@N', spin_id2='@H', direct_bond=True)
the routine gets blocked on encountering the spin with data:
  RelaxError: The magnetic dipole-dipole interaction already exists between
the spins '#1ogt-hkca_mol2:7@N' and '#1ogt-hkca_mol2:7@H'.
So either way it doesn't work and I'm stuck …
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