Apologies if this has been covered before - but can someone
          point me to a way of renumbering only specific chains (i.e. F,
          G, H etc).
          I have tried using phenix.pdbtools with either selection =
          "chain F or chain G or chain H" or keep "chain A or chain B or
          chain C or chain D" - but in both cases phenix renumbers every
          single residue!
        
        
        
        If you don't mind doing it interactively, try this:
        
        
        phenix.pdb_editor
        
        
        (also in the GUI, of course)  Still in development, but
          it's meant to be used for situations like this.  The big
          drawback of course is that you can't script it.
        
        
        Otherwise, what you're describing is very easy to code in
          Python, but it looks like pdbtools is simply not implemented
          this way.  One of us can change that (and possibly send you a
          patch, if it doesn't end up being too complex).
        
        
        -Nat