.. Friends Of Friends
   Matthieu Schaller 15th June 2019

.. _Fof_Parameter_Description_label:

Friends-Of-Friends Parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``FOF`` section of the parameter file contains all the options related
to the group finder. Some parameters only make sense in the stand-alone
version and some only in the on-the-fly version of the module.

The main parameter is the linking length that will be used. The easiest way
to define it is to set the ratio of the linking length to the mean *dark
matter* inter-particle separation (irrespective of whether there are other
kinds of particles present in the run). This is done using the parameter
``linking_length_ratio`` and the typical value used is
``0.2``. Users can optionally overwrite the linking length by imposing an
absolute value using the parameter ``absolute_linking_length``. This is
expressed in internal units. This value will be ignored (and the ratio of
the mean inter-particle separation will be used) when set to ``-1``.

The second important parameter is the minimal size of groups to retain in
the catalogues. This is given in terms of number of particles (of all types)
via the parameter ``min_group_size``. When analysing simulations, to
identify haloes, the common practice is to set this to ``32`` in order to
not plague the catalogue with too many small, likely unbound, structures.
When using the FOF code on-the-fly for black hole seeding, larger values
are recommended as there is no need to store groups much smaller than the
minimal halo mass (see below).

------------------------

In the case of black holes seeding, we run the FOF module on-the-fly during
a cosmological simulation. The time of the first FOF call is controlled by
the following two options:

  * Time of the first FOF call (non-cosmological runs): ``time_first``,
  * Scale-factor of the first FOF call (cosmological runs):
    ``scale_factor_first``.

One of those two parameters has to be provided depending on the type of
run. In the case of non-cosmological runs, the time of the first FOF call
is expressed in the internal units of time. Users also have to provide the
difference in time (or scale-factor) between consecutive outputs:

  * Time difference between consecutive outputs: ``delta_time``.

The last parameter relevant to the on-the-fly FOF module is the minimal
halo (group) mass considered for seeding black holes. This is specified by
the parameter ``black_hole_seed_halo_mass_Msun`` which is expressed in
solar masses.

------------------------

In the case of the stand-alone module, the four seeding parameters
described above are ignored but an additional one needs to be
specified. This is the name of the file in which the catalogue of groups will
be written. This is specified by the parameter ``fof_output``. The linking
length and minimal group to keep are set in the way described above.

There are two additional optional parameters that can be used to modify
details of the stand-alone FOF code. The ``GroupID`` of the particles that
do not belong to any groups is by default set to 2147483647
(i.e. :math:`2^{31}-1`). A different value can be specified by changing the
optional parameter ``group_id_default``. Similarly, the first group in the
catalogue (i.e. the largest group) carries the ``GroupID`` 1. This can be
changed by tweaking the optional parameter ``group_id_offset``.


------------------------

A full FOF section of the YAML parameter file looks like:

.. code:: YAML

    # Parameters of the Friends-Of-Friends module
    FOF:
       basename:                        fof_output  # Filename for the FOF outputs.
       scale_factor_first:              0.91        # Scale-factor of first FoF black hole seeding calls.
       time_first:                      0.2         # Time of first FoF black hole seeding calls.
       delta_time:                      1.005       # Time between consecutive FoF black hole seeding calls.
       min_group_size:                  256         # The minimum no. of particles required for a group.
       linking_length_ratio:            0.2         # Linking length in units of the main inter-particle separation.
       black_hole_seed_halo_mass_Msun:  1.5e10      # Minimal halo mass in which to seed a black hole (in solar masses).
       absolute_linking_length:         -1.         # (Optional) Absolute linking length (in internal units).
       group_id_default:                2147483647  # (Optional) Sets the group ID of particles in groups below the minimum size.
       group_id_offset:                 1           # (Optional) Sets the offset of group ID labelling. Defaults to 1 if unspecified.
