I was trying to clean two pdb files and superpose them based on the
pairwise structural alignment.
...
But when I replaced it to
'modeller.salign.iterative_structural_align(aln), I had error message like:
My best guess is that every combination of parameters failed for the
first loop, so it has no alignment to refine. If you can send me your
full inputs (which two PDBs are you trying to align?) I'll see if it can
be fixed.