def omega(CASRN, AvailableMethods=False, Method=None, IgnoreMethods=['<STR_LIT>', '<STR_LIT>']):
def list_methods():<EOL><INDENT>methods = []<EOL>if CASRN in _crit_PSRKR4.index and not np.isnan(_crit_PSRKR4.at[CASRN, '<STR_LIT>']):<EOL><INDENT>methods.append('<STR_LIT>')<EOL><DEDENT>if CASRN in _crit_PassutDanner.index and not np.isnan(_crit_PassutDanner.at[CASRN, '<STR_LIT>']):<EOL><INDENT>methods.append('<STR_LIT>')<EOL><DEDENT>if CASRN in _crit_Yaws.index and not np.isnan(_crit_Yaws.at[CASRN, '<STR_LIT>']):<EOL><INDENT>methods.append('<STR_LIT>')<EOL><DEDENT>Tcrit, Pcrit = Tc(CASRN), Pc(CASRN)<EOL>if Tcrit and Pcrit:<EOL><INDENT>if Tb(CASRN):<EOL><INDENT>methods.append('<STR_LIT>')<EOL><DEDENT>if VaporPressure(CASRN=CASRN).T_dependent_property(Tcrit*<NUM_LIT>):<EOL><INDENT>methods.append('<STR_LIT>')  <EOL><DEDENT><DEDENT>if IgnoreMethods:<EOL><INDENT>for Method in IgnoreMethods:<EOL><INDENT>if Method in methods:<EOL><INDENT>methods.remove(Method)<EOL><DEDENT><DEDENT><DEDENT>methods.append('<STR_LIT>')<EOL>return methods<EOL><DEDENT>if AvailableMethods:<EOL><INDENT>return list_methods()<EOL><DEDENT>if not Method:<EOL><INDENT>Method = list_methods()[<NUM_LIT:0>]<EOL><DEDENT>if Method == '<STR_LIT>':<EOL><INDENT>_omega = float(_crit_PSRKR4.at[CASRN, '<STR_LIT>'])<EOL><DEDENT>elif Method == '<STR_LIT>':<EOL><INDENT>_omega = float(_crit_PassutDanner.at[CASRN, '<STR_LIT>'])<EOL><DEDENT>elif Method == '<STR_LIT>':<EOL><INDENT>_omega = float(_crit_Yaws.at[CASRN, '<STR_LIT>'])<EOL><DEDENT>elif Method == '<STR_LIT>':<EOL><INDENT>_omega = LK_omega(Tb(CASRN), Tc(CASRN), Pc(CASRN))<EOL><DEDENT>elif Method == '<STR_LIT>':<EOL><INDENT>P = VaporPressure(CASRN=CASRN).T_dependent_property(Tc(CASRN)*<NUM_LIT>)<EOL>_omega = -log10(P/Pc(CASRN)) - <NUM_LIT:1.0><EOL><DEDENT>elif Method == '<STR_LIT>':<EOL><INDENT>_omega = None<EOL><DEDENT>else:<EOL><INDENT>raise Exception('<STR_LIT>')<EOL><DEDENT>return _omega<EOL>
r'''This function handles the retrieval of a chemical's acentric factor,
    `omega`, or its calculation from correlations or directly through the
    definition of acentric factor if possible. Requires a known boiling point,
    critical temperature and pressure for use of the correlations. Requires
    accurate vapor pressure data for direct calculation.

    Will automatically select a method to use if no Method is provided;
    returns None if the data is not available and cannot be calculated.

    .. math::
        \omega \equiv -\log_{10}\left[\lim_{T/T_c=0.7}(P^{sat}/P_c)\right]-1.0

    Examples
    --------
    >>> omega(CASRN='64-17-5')
    0.635

    Parameters
    ----------
    CASRN : string
        CASRN [-]

    Returns
    -------
    omega : float
        Acentric factor of compound
    methods : list, only returned if AvailableMethods == True
        List of methods which can be used to obtain omega with the given inputs

    Other Parameters
    ----------------
    Method : string, optional
        The method name to use. Accepted methods are 'PSRK', 'PD', 'YAWS', 
        'LK', and 'DEFINITION'. All valid values are also held in the list
        omega_methods.
    AvailableMethods : bool, optional
        If True, function will determine which methods can be used to obtain
        omega for the desired chemical, and will return methods instead of
        omega
    IgnoreMethods : list, optional
        A list of methods to ignore in obtaining the full list of methods,
        useful for for performance reasons and ignoring inaccurate methods

    Notes
    -----
    A total of five sources are available for this function. They are:

        * 'PSRK', a compillation of experimental and estimated data published 
          in the Appendix of [15]_, the fourth revision of the PSRK model.
        * 'PD', an older compillation of
          data published in (Passut & Danner, 1973) [16]_.
        * 'YAWS', a large compillation of data from a
          variety of sources; no data points are sourced in the work of [17]_.
        * 'LK', a estimation method for hydrocarbons.
        * 'DEFINITION', based on the definition of omega as
          presented in [1]_, using vapor pressure data.

    References
    ----------
    .. [1] Pitzer, K. S., D. Z. Lippmann, R. F. Curl, C. M. Huggins, and
       D. E. Petersen: The Volumetric and Thermodynamic Properties of Fluids.
       II. Compressibility Factor, Vapor Pressure and Entropy of Vaporization.
       J. Am. Chem. Soc., 77: 3433 (1955).
    .. [2] Horstmann, Sven, Anna Jabłoniec, Jörg Krafczyk, Kai Fischer, and
       Jürgen Gmehling. "PSRK Group Contribution Equation of State:
       Comprehensive Revision and Extension IV, Including Critical Constants
       and Α-Function Parameters for 1000 Components." Fluid Phase Equilibria
       227, no. 2 (January 25, 2005): 157-64. doi:10.1016/j.fluid.2004.11.002.
    .. [3] Passut, Charles A., and Ronald P. Danner. "Acentric Factor. A
       Valuable Correlating Parameter for the Properties of Hydrocarbons."
       Industrial & Engineering Chemistry Process Design and Development 12,
       no. 3 (July 1, 1973): 365-68. doi:10.1021/i260047a026.
    .. [4] Yaws, Carl L. Thermophysical Properties of Chemicals and
       Hydrocarbons, Second Edition. Amsterdam Boston: Gulf Professional
       Publishing, 2014.