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

        #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)