def _build_phonon_supercell(self):
        """
        phonon_supercell:
          This supercell is used for harmonic phonons (frequencies,
          eigenvectors, group velocities, ...)
        phonon_supercell_matrix:
          Different supercell size can be specified.
        """
        if self._phonon_supercell_matrix is None:
            self._phonon_supercell = self._supercell
        else:
            self._phonon_supercell = get_supercell(
                self._unitcell, self._phonon_supercell_matrix, self._symprec)