def Ihmels(Tc=None, Pc=None, Vc=None):
    r'''Most recent, and most recommended method of estimating critical
    properties from each other. Two of the three properties are required.
    This model uses the "critical surface", a general plot of Tc vs Pc vs Vc.
    The model used 421 organic compounds to derive equation.
    The general equation is in [1]_:

    .. math::
        P_c = -0.025 + 2.215 \frac{T_c}{V_c}

    Parameters
    ----------
    Tc : float
        Critical temperature of fluid (optional) [K]
    Pc : float
        Critical pressure of fluid (optional) [Pa]
    Vc : float
        Critical volume of fluid (optional) [m^3/mol]

    Returns
    -------
    Tc, Pc or Vc : float
        Critical property of fluid [K], [Pa], or [m^3/mol]

    Notes
    -----
    The prediction of Tc from Pc and Vc is not tested, as this is not necessary
    anywhere, but it is implemented.
    Internal units are MPa, cm^3/mol, and K. A slight error occurs when
    Pa, cm^3/mol and K are used instead, on the order of <0.2%.
    Their equation was also compared with 56 inorganic and elements.
    Devations of 20% for <200K or >1000K points.

    Examples
    --------a
    Succinic acid [110-15-6]

    >>> Ihmels(Tc=851.0, Vc=0.000308)
    6095016.233766234

    References
    ----------
    .. [1] Ihmels, E. Christian. "The Critical Surface." Journal of Chemical
           & Engineering Data 55, no. 9 (September 9, 2010): 3474-80.
           doi:10.1021/je100167w.
    '''
    if Tc and Vc:
        Vc = Vc*1E6  # m^3/mol to cm^3/mol
        Pc = -0.025+2.215*Tc/Vc
        Pc = Pc*1E6  # MPa to Pa
        return Pc
    elif Tc and Pc:
        Pc = Pc/1E6  # Pa to MPa
        Vc = 443*Tc/(200*Pc+5)
        Vc = Vc/1E6  # cm^3/mol to m^3/mol
        return Vc
    elif Pc and Vc:
        Pc = Pc/1E6  # Pa to MPa
        Vc = Vc*1E6  # m^3/mol to cm^3/mol
        Tc = 5.0/443*(40*Pc*Vc + Vc)
        return Tc
    else:
        raise Exception('Two of Tc, Pc, and Vc must be provided')
r'''Most recent, and most recommended method of estimating critical
    properties from each other. Two of the three properties are required.
    This model uses the "critical surface", a general plot of Tc vs Pc vs Vc.
    The model used 421 organic compounds to derive equation.
    The general equation is in [1]_:

    .. math::
        P_c = -0.025 + 2.215 \frac{T_c}{V_c}

    Parameters
    ----------
    Tc : float
        Critical temperature of fluid (optional) [K]
    Pc : float
        Critical pressure of fluid (optional) [Pa]
    Vc : float
        Critical volume of fluid (optional) [m^3/mol]

    Returns
    -------
    Tc, Pc or Vc : float
        Critical property of fluid [K], [Pa], or [m^3/mol]

    Notes
    -----
    The prediction of Tc from Pc and Vc is not tested, as this is not necessary
    anywhere, but it is implemented.
    Internal units are MPa, cm^3/mol, and K. A slight error occurs when
    Pa, cm^3/mol and K are used instead, on the order of <0.2%.
    Their equation was also compared with 56 inorganic and elements.
    Devations of 20% for <200K or >1000K points.

    Examples
    --------a
    Succinic acid [110-15-6]

    >>> Ihmels(Tc=851.0, Vc=0.000308)
    6095016.233766234

    References
    ----------
    .. [1] Ihmels, E. Christian. "The Critical Surface." Journal of Chemical
           & Engineering Data 55, no. 9 (September 9, 2010): 3474-80.
           doi:10.1021/je100167w.