code: def filesfile_string(self):
          
          lines = []
          app = lines.append
  
          app(self.input_file.path)          # 1) Path of the input file
          app(self.output_file.path)         # 2) Path of the output file
          app(self.ddb_filepath)             # 3) Input derivative database e.g. t13.ddb.in
          app(self.md_filepath)              # 4) Output molecular dynamics e.g. t13.md
          app(self.gkk_filepath)             # 5) Input elphon matrix elements  (GKK file)
          app(self.outdir.path_join("out"))  # 6) Base name for elphon output files e.g. t13
          app(self.ddk_filepath)             # 7) File containing ddk filenames for elphon/transport.
  
          return "\n".join(lines)
docs: String with the list of files and prefixes needed to execute ABINIT.