How to use the pynbody.snapshot.gadgethdf.GadgetHDFSnap.derived_quantity function in pynbody

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def NV(sim) :
    """Fraction of Nitrogen NV"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='nv')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def HeII(sim) :
    """Fraction of Helium HeII"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='heii')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def doppler_redshift(sim) :
    """Doppler Redshift from LoS Velocity 'losvel' using SR """

    return np.sqrt( (1. + sim['losvel'].in_units('c')) / (1. - sim['losvel'].in_units('c'))  ) - 1.

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def CIV(sim) :
    """Fraction of Carbon CIV"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='civ')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def cosmodm(sim) :
    """Cosmological Dispersion measure per SPH particle includes (1+z) factor, currently ignoring n_e contribution from He """

    return sim.g["rho_ne"] * (1. + sim.g["redshift"])
@GadgetHDFSnap.derived_quantity

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def MGII(sim) :
    """Fraction of Magnesium MgII"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='mgii')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def em(sim) :
    """Emission Measure (n_e^2) per particle to be integrated along LoS"""

    return sim.g["rho_ne"]*sim.g["rho_ne"]

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def c_n_sq(sim) :
    """Turbulent amplitude C_N^2 for use in SM calculations (e.g. Eqn 20 of Macquart & Koay 2013 ApJ 776 2) """

    ## Spectrum of turbulence below the SPH resolution, assume Kolmogorov
    beta = 11./3.
    L_min = 0.1*units.Mpc
    c_n_sq = ((beta - 3.)/((2.)*(2.*np.pi)**(4.-beta)))*L_min**(3.-beta)*sim["em"]
    c_n_sq.units = units.m**(-20,3)

    return c_n_sq

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def sxh(self) :
    minsx = np.amin(self['S'][np.where(self['S'] > 0)])
    self['S'][np.where(self['S'] == 0)]=minsx
    return np.log10(self['S']/self['S']) - np.log10(XSOLS/XSOLH)

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def ofe(self) :
    minox = np.amin(self['O'][np.where(self['O'] > 0)])
    self['O'][np.where(self['O'] == 0)]=minox
    minfe = np.amin(self['Fe'][np.where(self['Fe'] > 0)])
    self['Fe'][np.where(self['Fe'] == 0)]=minfe
    return np.log10(self['O']/self['Fe']) - np.log10(XSOLO/XSOLFe)