# Stand-Alone Friends-Of-Friends¶

The stand-alone version of the Friends-Of-Friends module can be compiled by configuring the code with the option --enable-stand-alone-fof. The fof and fof_mpi executables will then be generated alongside the regular SWIFT ones.

The executable takes a parameter file as an argument. It will then read the snapshot specified in the parameter file and extract all the dark matter particles by default. FOF is then run on these particles and a catalogue of groups is written to disk. Additional particle types can be read and processed by the stand-alone FOF code by adding any of the following runtime parameters to the command line:

• --hydro: Read and process the gas particles,
• --stars: Read and process the star particles,
• --black-holes: Read and process the black hole particles.

The group catalogues contain a unique identifier for each group as well as the number of particles in each group and their total mass (in internal units). The FOF code will also write a snapshot with an additional field for each particle. This contains the GroupID of each particle and can be used to find all the particles in a given halo and to link them to the information stored in the catalogue.