Restart the structural relaxation.

        Structure relaxations can be restarted only if we have the WFK file or the DEN or the GSR file
        from which we can read the last structure (mandatory) and the wavefunctions (not mandatory but useful).
        Prefer WFK over other files since we can reuse the wavefunctions.

        .. note::

            The problem in the present approach is that some parameters in the input
            are computed from the initial structure and may not be consistent with
            the modification of the structure done during the structure relaxation.

def restart(self):
        """
        Restart the structural relaxation.

        Structure relaxations can be restarted only if we have the WFK file or the DEN or the GSR file
        from which we can read the last structure (mandatory) and the wavefunctions (not mandatory but useful).
        Prefer WFK over other files since we can reuse the wavefunctions.

        .. note::

            The problem in the present approach is that some parameters in the input
            are computed from the initial structure and may not be consistent with
            the modification of the structure done during the structure relaxation.
        """
        restart_file = None

        # Try to restart from the WFK file if possible.
        # FIXME: This part has been disabled because WFK=IO is a mess if paral_kgb == 1
        # This is also the reason why I wrote my own MPI-IO code for the GW part!
        wfk_file = self.outdir.has_abiext("WFK")
        if False and wfk_file:
            irdvars = irdvars_for_ext("WFK")
            restart_file = self.out_to_in(wfk_file)

        # Fallback to DEN file. Note that here we look for out_DEN instead of out_TIM?_DEN
        # This happens when the previous run completed and task.on_done has been performed.
        # ********************************************************************************
        # Note that it's possible to have an undetected error if we have multiple restarts
        # and the last relax died badly. In this case indeed out_DEN is the file produced
        # by the last run that has executed on_done.
        # ********************************************************************************
        if restart_file is None:
            for ext in ("", ".nc"):
                out_den = self.outdir.path_in("out_DEN" + ext)
                if os.path.exists(out_den):
                    irdvars = irdvars_for_ext("DEN")
                    restart_file = self.out_to_in(out_den)
                    break

        if restart_file is None:
            # Try to restart from the last TIM?_DEN file.
            # This should happen if the previous run didn't complete in clean way.
            # Find the last TIM?_DEN file.
            last_timden = self.outdir.find_last_timden_file()
            if last_timden is not None:
                if last_timden.path.endswith(".nc"):
                    ofile = self.outdir.path_in("out_DEN.nc")
                else:
                    ofile = self.outdir.path_in("out_DEN")
                os.rename(last_timden.path, ofile)
                restart_file = self.out_to_in(ofile)
                irdvars = irdvars_for_ext("DEN")

        if restart_file is None:
            # Don't raise RestartError as we can still change the structure.
            self.history.warning("Cannot find the WFK|DEN|TIM?_DEN file to restart from.")
        else:
            # Add the appropriate variable for restarting.
            self.set_vars(irdvars)
            self.history.info("Will restart from %s", restart_file)

        # FIXME Here we should read the HIST file but restartxf if broken!
        #self.set_vars({"restartxf": -1})

        # Read the relaxed structure from the GSR file and change the input.
        self._change_structure(self.get_final_structure())

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