def phase_identification_parameter(V, dP_dT, dP_dV, d2P_dV2, d2P_dVdT):
return V*(d2P_dVdT/dP_dT - d2P_dV2/dP_dV)<EOL>
r'''Calculate the Phase Identification Parameter developed in [1]_ for
    the accurate and efficient determination of whether a fluid is a liquid or
    a gas based on the results of an equation of state. For supercritical 
    conditions, this provides a good method for choosing which property 
    correlations to use.

    .. math::
        \Pi = V \left[\frac{\frac{\partial^2 P}{\partial V \partial T}}
        {\frac{\partial P }{\partial T}}- \frac{\frac{\partial^2 P}{\partial 
        V^2}}{\frac{\partial P}{\partial V}} \right]

    Parameters
    ----------
    V : float
        Molar volume at `T` and `P`, [m^3/mol]
    dP_dT : float
        Derivative of `P` with respect to `T`, [Pa/K]
    dP_dV : float
        Derivative of `P` with respect to `V`, [Pa*mol/m^3]
    d2P_dV2 : float
        Second derivative of `P` with respect to `V`, [Pa*mol^2/m^6]
    d2P_dVdT : float
        Second derivative of `P` with respect to both `V` and `T`, [Pa*mol/m^3/K]

    Returns
    -------
    PIP : float
        Phase Identification Parameter, [-]

    Notes
    -----
    Heuristics were used by process simulators before the invent of this 
    parameter. 

    The criteria for liquid is Pi > 1; for vapor, Pi <= 1.

    There is also a solid phase mechanism available. For solids, the Solid  
    Phase Identification Parameter is greater than 1, like liquids; however,  
    unlike liquids, d2P_dVdT is always >0; it is < 0 for liquids and gases.

    Examples
    --------
    Calculated for hexane from the PR EOS at 299 K and 1 MPa (liquid):

    >>> phase_identification_parameter(0.000130229900874, 582169.397484, 
    ... -3.66431747236e+12, 4.48067893805e+17, -20518995218.2)
    11.33428990564796

    References
    ----------
    .. [1] Venkatarathnam, G., and L. R. Oellrich. "Identification of the Phase
       of a Fluid Using Partial Derivatives of Pressure, Volume, and 
       Temperature without Reference to Saturation Properties: Applications in 
       Phase Equilibria Calculations." Fluid Phase Equilibria 301, no. 2 
       (February 25, 2011): 225-33. doi:10.1016/j.fluid.2010.12.001.
    .. [2] Jayanti, Pranava Chaitanya, and G. Venkatarathnam. "Identification
       of the Phase of a Substance from the Derivatives of Pressure, Volume and
       Temperature, without Prior Knowledge of Saturation Properties: Extension
       to Solid Phase." Fluid Phase Equilibria 425 (October 15, 2016): 269-277.
       doi:10.1016/j.fluid.2016.06.001.