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6b. Fe\ :sup:`3+`\ /Fe\ :sub:`T` to *f*\ \ :sub:`O2` dependence
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To add a new parameterisation for Fe\ :sup:`3+`\ /Fe\ :sub:`T` to *f*\ \ :sub:`O2` dependence involves creating a new “terms” function, which details how any parameters depend on pressure, temperature, melt composition, etc.

Here is an example for Borisov et al. (2018) in ``model_dependent_variables.py``:

.. code-block:: python

    def FefO2_Borisov18_terms(
        PT, melt_wf, models=default_models
        ):  # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
        T_K = PT["T"] + 273.15
        # Borisov et al. (2018) CMP 173
        a = 0.207
        # melt mole fraction with no volatiles and all Fe as FeOT
        melt_comp = mg.melt_mole_fraction(
            melt_wf, models, "no", "no", molmass="ONeill22", majors="ONeill22"
        )
        B = (
            4633.3 / T_K
            - 0.445 * melt_comp["SiO2"]
            - 0.900 * melt_comp["TiO2"]
            + 1.532 * melt_comp["MgO"]
            + 0.314 * melt_comp["CaO"]
            + 2.030 * melt_comp["Na2O"]
            + 3.355 * melt_comp["K2O"]
            - 4.851 * melt_comp["P2O5"]
            - 3.081 * melt_comp["SiO2"] * melt_comp["Al2O3"]
            - 4.370 * melt_comp["SiO2"] * melt_comp["MgO"]
            - 1.852
        )
        return a, B

Then, the new parameterisation must be added to the functions ``fO22Fe3FeT()`` and ``f_O2()`` in ``model_dependent_variables.py``, which is shown here for Borisov et al. (2018):

.. code-block:: python

    def fO22Fe3FeT(fO2, PT, melt_wf, models=default_models):  # converting fO2 to Fe3/FeT

        model = models.loc["fO2", "option"]

        # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
        if model == "Borisov18":
            a, B = FefO2_Borisov18_terms(PT, melt_wf, models)
            Fe3Fe2 = 10.0 ** (a * math.log10(fO2) + B)
            return Fe3Fe2 / (Fe3Fe2 + 1.0)

    def f_O2(PT, melt_wf, models=default_models):

        model = models.loc["fO2", "option"]

        # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
        elif model == "Borisov18":
            F = mg.Fe3Fe2(melt_wf)
            a, B = FefO2_Borisov18_terms(PT, melt_wf, models)
            fO2 = 10.0 ** ((math.log10(F) - B) / a)
            return fO2

To use it, the "fO2" option in "models" would be set to "Borisov18".