code: def filesfile_string(self):
          
          lines = []
          app = lines.append
  
          #optic.in     ! Name of input file
          #optic.out    ! Unused
          #optic        ! Root name for all files that will be produced
          app(self.input_file.path)                           # Path to the input file
          app(os.path.join(self.workdir, "unused"))           # Path to the output file
          app(os.path.join(self.workdir, self.prefix.odata))  # Prefix for output data
  
          return "\n".join(lines)
docs: String with the list of files and prefixes needed to execute ABINIT.