It takes a value of seebeck and adjusts the analytic seebeck until it's equal
        Returns: eta where the two seebeck coefficients are equal 
        (reduced chemical potential)

def eta_from_seebeck(seeb,Lambda):
    """ 
        It takes a value of seebeck and adjusts the analytic seebeck until it's equal
        Returns: eta where the two seebeck coefficients are equal 
        (reduced chemical potential)
    """
    from scipy.optimize import fsolve
    out = fsolve(lambda x: (seebeck_spb(x,Lambda) - abs(seeb)) ** 2, 1.,full_output=True)
    return out[0][0]