def calc_orientation_vector(rl_vectors, orientation):
    """
    Use the oriented reflection (hkl) to calculate an orientation vector for
    crystal.

    Args:
        rl_vectors (ndarray): reciprocal lattice vectors.
        orientation (ndarray): hkl indices of reflection normal to surface.

    Returns:
        g_hkl (ndarray): the orientation vector of the crystal.
    """
    rl_a = rl_vectors[0]
    rl_b = rl_vectors[1]
    rl_c = rl_vectors[2]
    g_hkl = orientation[0] * rl_a + orientation[1] * rl_b + orientation[2] * \
        rl_c
    return g_hkl