def set_permutation_symmetry(force_constants):
    """Enforce permutation symmetry to force cosntants by

    Phi_ij_ab = Phi_ji_ba

    i, j: atom index
    a, b: Cartesian axis index

    This is not necessary for harmonic phonon calculation because this
    condition is imposed when making dynamical matrix Hermite in
    dynamical_matrix.py.

    """
    fc_copy = force_constants.copy()
    for i in range(force_constants.shape[0]):
        for j in range(force_constants.shape[1]):
            force_constants[i, j] = (force_constants[i, j] + fc_copy[j, i].T
                ) / 2
