def molar_refractivity_from_RI(RI, Vm):
    """
    Returns the molar refractivity of a fluid given its molar volume and
    refractive index.

    .. math::
        R_m = \\left(\\frac{n^2-1}{n^2+2}\\right)V_m

    Parameters
    ----------
    RI : float
        Refractive Index on Na D line, [-]
    Vm : float
        Molar volume of fluid, [m^3/mol]

    Returns
    -------
    Rm : float
        Molar refractivity [m^3/mol]

    Notes
    -----

    Examples
    --------
    >>> molar_refractivity_from_RI(1.3611, 5.8676E-5)
    1.2985217089649597e-05

    References
    ----------
    .. [1] Panuganti, Sai R., Fei Wang, Walter G. Chapman, and Francisco M.
       Vargas. "A Simple Method for Estimation of Dielectric Constants and
       Polarizabilities of Nonpolar and Slightly Polar Hydrocarbons."
       International Journal of Thermophysics 37, no. 7 (June 6, 2016): 1-24.
       doi:10.1007/s10765-016-2075-8.
    """
    return (RI ** 2 - 1.0) / (RI ** 2 + 2.0) * Vm
