code: def _build_phonon_supercell(self):
          
          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)
docs: phonon_supercell:
            This supercell is used for harmonic phonons (frequencies,
            eigenvectors, group velocities, ...)
          phonon_supercell_matrix:
            Different supercell size can be specified.