def force_factor(rij2):
    """Lennard-Jones force magnitudefd exerted by atom j on atom i.

    :param float|np.array rij2: squared interatomic distance from atom i to atom j
    :return: fij
    """
    rm2 = 1.0/rij2
    rm6 = (rm2*rm2*rm2)
    f = (1.0 - 2.0*rm6 )*rm6*rm2*6.0
    return f