def Grigoras(Tc=None, Pc=None, Vc=None):
    r'''Relatively recent (1990) relationship for 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 137 organic and inorganic compounds to derive the equation.
    The general equation is in [1]_:

    .. math::
        P_c = 2.9 + 20.2 \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 bar, 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%.
    This equation is less accurate than that of Ihmels, but surprisingly close.
    The author also investigated an early QSPR model.

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

    >>> Grigoras(Tc=851.0, Vc=0.000308)
    5871233.766233766

    References
    ----------
    .. [1] Grigoras, Stelian. "A Structural Approach to Calculate Physical
           Properties of Pure Organic Substances: The Critical Temperature,
           Critical Volume and Related Properties." Journal of Computational
           Chemistry 11, no. 4 (May 1, 1990): 493-510.
           doi:10.1002/jcc.540110408
    '''
    if Tc and Vc:
        Vc = Vc*1E6  # m^3/mol to cm^3/mol
        Pc = 2.9 + 20.2*Tc/Vc
        Pc = Pc*1E5  # bar to Pa
        return Pc
    elif Tc and Pc:
        Pc = Pc/1E5  # Pa to bar
        Vc = 202.0*Tc/(10*Pc-29.0)
        Vc = Vc/1E6  # cm^3/mol to m^3/mol
        return Vc
    elif Pc and Vc:
        Pc = Pc/1E5  # Pa to bar
        Vc = Vc*1E6  # m^3/mol to cm^3/mol
        Tc = 1.0/202*(10*Pc-29.0)*Vc
        return Tc
    else:
        raise Exception('Two of Tc, Pc, and Vc must be provided')
r'''Relatively recent (1990) relationship for 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 137 organic and inorganic compounds to derive the equation.
    The general equation is in [1]_:

    .. math::
        P_c = 2.9 + 20.2 \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 bar, 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%.
    This equation is less accurate than that of Ihmels, but surprisingly close.
    The author also investigated an early QSPR model.

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

    >>> Grigoras(Tc=851.0, Vc=0.000308)
    5871233.766233766

    References
    ----------
    .. [1] Grigoras, Stelian. "A Structural Approach to Calculate Physical
           Properties of Pure Organic Substances: The Critical Temperature,
           Critical Volume and Related Properties." Journal of Computational
           Chemistry 11, no. 4 (May 1, 1990): 493-510.
           doi:10.1002/jcc.540110408