String with the list of files and prefixes needed to execute ABINIT.

def filesfile_string(self):
        """String with the list of files and prefixes needed to execute ABINIT."""
        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)