how could I carry out GA341 assessment on a model not produced by
MODELLER but by an ab initio method?
I'm using MODELLER 9v2.
The example given at http://salilab.org/modeller/9v4/manual/node188.htmlshould work for any PDB file - not just one produced by Modeller. But,
as described in the manual, the sequence identity is a parameter used in
the GA341 score, so you need to provide that. For models that are not
homology-derived, you'll have to make a reasonable guess at some
"equivalent sequence identity" or use a different assessment method,
such as DOPE.