Clarified the option to read the metal information from the ICs in the EAGLE...

Clarified the option to read the metal information from the ICs in the EAGLE chemistry model documentation.

doc/RTD/source/SubgridModels/EAGLE/index.rst

@@ -143,6 +143,37 @@ Whilst one would use the following values for solar abundances
      init_abundance_Silicon:      6.825874e-4  # Mass fraction in Silicon
      init_abundance_Iron:         1.1032152e-3 # Mass fraction in Iron
 
+Individual element abundances for each particle can also be read
+directly from the ICs. By default these are overwritten in the code by
+the values read from the YAML file. However, users can set the
+parameter ``init_abundance_metal`` to ``-1`` to make SWIFT ignore the
+values provided in the parameter file:
+
+.. code:: YAML
+
+   EAGLEChemistry:
+     init_abundance_metal:       -1     # Read the particles' metal mass fractions from the ICs.
+
+
+The ICs must then contain values for these three fields (same as what
+is written to the snapshots):
+
++----------------------------------+-------------------------------------+-----------+-----------------------------+
+| Name                             | Description                         | Units     | Comments                    |
++==================================+=====================================+===========+=============================+
+| ``ElementAbundance``             | | Fraction of the gas/star mass     | [-]       | | Array of length           |
+|                                  | | in the different elements         |           | | 9 for each particle       |
++----------------------------------+-------------------------------------+-----------+-----------------------------+
+| ``Metallicity``                  | | Fraction of the gas/star mass     | [-]       |                             |
+|                                  | | in *all* metals                   |           |                             |
++----------------------------------+-------------------------------------+-----------+-----------------------------+
+| ``IronMassFracFromSNIa``         | | Fraction of the *total* gas/star  | [-]       |                             |
+|                                  | | mass in *iron* produced produced  |           |                             |
+|                                  | | by enrichment from SNIa stars     |           |                             |
++----------------------------------+-------------------------------------+-----------+-----------------------------+
+
+If these fields are absent, then a value of ``0`` will be used for all
+of them, likely leading to issues in the way the code will run.
 
 .. _EAGLE_cooling: