def WLF(Temp, RefT, WLF_C1, WLF_C2):
    """
    Calculate the Williams-Landel-Ferry (WLF) equation [1].
    
    Parameters
    ----------
    Temp : numeric
        Evaluation temperature of the shift factor.
    RefT : numeric
        Reference temperature chosen to construct the master curve.
    WLF_C1, WLF_C2 : numeric
        Empirical constants. (Obtained from fitting the shift factor a_T)

    Returns
    -------
    log_aT : numeric
        The decadic logarithm of the WLF shift factor.

    References
    ----------
    [1] Williams, Malcolm L.; Landel, Robert F.; Ferry, John D. (1955). 
    "The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers 
    and Other Glass-forming Liquids". J. Amer. Chem. Soc. 77 (14): 3701-3707. 
    doi:10.1021/ja01619a008
    """
    log_aT = -WLF_C1*(Temp - RefT)/(WLF_C2+(Temp - RefT))
    return log_aT