def Wagner(T, Tc, Pc, a, b, c, d):
Tr = T/Tc<EOL>tau = <NUM_LIT:1.0> - T/Tc<EOL>return Pc*exp((a*tau + b*tau**<NUM_LIT> + c*tau**<NUM_LIT> + d*tau**<NUM_LIT:5>)/Tr)<EOL>
r'''Calculates vapor pressure using the Wagner equation (2.5, 5 form).

    Requires critical temperature and pressure as well as four coefficients
    specific to each chemical.

    .. math::
        \ln P^{sat}= \ln P_c + \frac{a\tau + b \tau^{1.5} + c\tau^{2.5}
        + d\tau^5} {T_r}

        \tau = 1 - \frac{T}{T_c}

    Parameters
    ----------
    T : float
        Temperature of fluid, [K]
    Tc : float
        Critical temperature, [K]
    Pc : float
        Critical pressure, [Pa]
    a, b, c, d : floats
        Parameters for wagner equation. Specific to each chemical. [-]

    Returns
    -------
    Psat : float
        Vapor pressure at T [Pa]

    Notes
    -----
    Warning: Pc is often treated as adjustable constant.

    Examples
    --------
    Methane, coefficients from [2]_, at 100 K.

    >>> Wagner(100., 190.551, 4599200, -6.02242, 1.26652, -0.5707, -1.366)
    34415.00476263708

    References
    ----------
    .. [1] Wagner, W. "New Vapour Pressure Measurements for Argon and Nitrogen and
       a New Method for Establishing Rational Vapour Pressure Equations."
       Cryogenics 13, no. 8 (August 1973): 470-82. doi:10.1016/0011-2275(73)90003-9
    .. [2] Poling, Bruce E. The Properties of Gases and Liquids. 5th edition.
       New York: McGraw-Hill Professional, 2000.