DFPT calculations can be restarted only if we have the 1WF file or the 1DEN file.
        from which we can read the first-order wavefunctions or the first order density.
        Prefer 1WF over 1DEN since we can reuse the wavefunctions.

def restart(self):
        """
        DFPT calculations can be restarted only if we have the 1WF file or the 1DEN file.
        from which we can read the first-order wavefunctions or the first order density.
        Prefer 1WF over 1DEN since we can reuse the wavefunctions.
        """
        # Abinit adds the idir-ipert index at the end of the file and this breaks the extension
        # e.g. out_1WF4, out_DEN4. find_1wf_files and find_1den_files returns the list of files found
        restart_file, irdvars = None, None

        # Highest priority to the 1WF file because restart is more efficient.
        wf_files = self.outdir.find_1wf_files()
        if wf_files is not None:
            restart_file = wf_files[0].path
            irdvars = irdvars_for_ext("1WF")
            if len(wf_files) != 1:
                restart_file = None
                self.history.critical("Found more than one 1WF file in outdir. Restart is ambiguous!")

        if restart_file is None:
            den_files = self.outdir.find_1den_files()
            if den_files is not None:
                restart_file = den_files[0].path
                irdvars = {"ird1den": 1}
                if len(den_files) != 1:
                    restart_file = None
                    self.history.critical("Found more than one 1DEN file in outdir. Restart is ambiguous!")

        if restart_file is None:
            # Raise because otherwise restart is equivalent to a run from scratch --> infinite loop!
            raise self.RestartError("%s: Cannot find the 1WF|1DEN file to restart from." % self)

        # Move file.
        self.history.info("Will restart from %s", restart_file)
        restart_file = self.out_to_in(restart_file)

        # Add the appropriate variable for restarting.
        self.set_vars(irdvars)

        # Now we can resubmit the job.
        return self._restart()