Config.sh Options

A number of features of AREPO are controlled with compile-time options in the makefile rather than by the parameter file. This has been done in order to allow the generation of highly optimised binaries by the compiler, even when the underlying source code allows for many different ways to run the code. The Template-Config.sh file contains a list of all available compile-time options, with most of them commented out by default. Below, a brief guide to these options is given.

Note

This page documents Template-Config.sh options which are not considered part of any non-standard module. Documentation for module specific compile-time options can be found under their respective pages.

Basic operating modes 

NTYPES=6: The number of distinct particle types used. No longer hard-coded to six.
REFLECTIVE_X/Y/Z=1: Sets non-periodic boundary conditions for the X/Y/Z boundary. A value of 1 makes a boundary a reflective boundary. A value of 2 sets inflow/outflow boundaries.
LONG_X/Y/Z=2.: These options can be used to distort the periodic simulation cube along the given direction with the given factor into a parallelepiped of arbitrary aspect ratio. The box size in the given direction increases from the value in the parameterfile by the factor given (e.g. if Boxsize is set to 100. and LONG_X=4. is set the simulation domain extends from 0 to 400 along X and from 0 to 100 along Y and Z.)
TWODIMS: This effectively switches off one dimension, i.e. the code follows only 2d hydrodynamics in the xy-plane. This only works with NOGRAVITY, and if all coordinates of the third axis are exactly equal. Can be useful for idealized tests.
ONEDIMS: This switches to 1D. The only valid spatial coordinate is the first dimension. All values of the other two coordinates have to be zero. For 1D simulations, refinement and derefinement are not supported, nor is MPI parallelization. The code is in this case fully OpenMP parallel.
ONEDIMS_SPHERICAL: This can be used in addition to ONEDIMS to obtain 1D spherical coordinates. The first dimension is used as the radial coordinate.
NOHYDRO: This turns off hydrodynamics, effectively running a gravity-only simulation.

Gravity treatment 

SELFGRAVITY: Computes pairwise gravitational interactions between simulation particles and cells.
HIERARCHICAL_GRAVITY: Uses hierarchical splitting of the time integration of the gravity.
NO_GAS_SELFGRAVITY: Switches off gas self-gravity in the tree.
GRAVITY_NOT_PERIODIC: Controls whether gravity is periodic. Further, PMGRID can be used in non-periodic mode as well for the gravity.

Tree gravity & softening 

In the default configuration, the code uses a small table of possible gravitational softening lengths, which are specified in the parameterfile through the SofteningComovingTypeX and SofteningMaxPhysTypeX options, where X is an integer that gives the “softening type”. Each particle type is mapped to one of these softening types through the SofteningTypeOfPartTypeY parameters, where Y gives the particle type. The number of particle types and the number of softening types do not necessarily have to be equal (although the default for both of them is 6). Several particle types can be mapped to the same softening if desired.

ADAPTIVE_HYDRO_SOFTENING: When this is enabled, the gravitational softening lengths of hdyro cells are varied according to their radius. To this end, the radius of a cell is multiplied by the parameter GasSoftFactor. Then, the closest softening from a logarithmicaly spaced table of possible softenings is adopted for the cell. The minimum softening in the table is specified by the parameter MinimumComovingHydroSoftening, and the larger ones are spaced a factor AdaptiveHydroSofteningSpacing apart. The resulting minimum and maximum softening values are reported in the stdout log file. Generally recommended for hydro simulations.
MULTIPLE_NODE_SOFTENING: If the tree walk wants to use a ‘softened node’ (i.e. where the maximum gravitational softening of some particles in the node is larger than the node distance and larger than the target particle’s softening), the node is opened by default (because there could be mass components with a still smaller softening hidden in the node). This can cause a subtantial performance penalty in some cases. By setting this option, this can be avoided. The code will then be allowed to use softened nodes, but it does that by evaluating the node-particle interaction for each mass component with different softening type separately (but by neglecting possible shifts in their centers of masses). This also requires that each tree node computes and stores a vector with these different masses. It is therefore desirable to not make the table of softening types excessively large. This option can be combined with adaptive hydro softening. In this case, particle type 0 needs to be mapped to softening type 0 in the parameterfile, and no other particle type may be mapped to softening type 0 (the code will issue an error message if one doesn’t obey to this). Generally recommended for hydro simulations.
INDIVIDUAL_GRAVITY_SOFTENING=2+4: The code can also be asked to set the softening types of some of the particle types automatically based on particle mass. The particle types to which this is applied are set by this compile time option through a bitmask encoding the types. The code by default assumes that the softening of particle type 1 should be the reference. To this end, the code determines the average mass of type 1 particles, and the types selected through this option then compute a desired softening length by scaling the type-1 softening with the cube root of the mass ratio. Then, the softening type that is closest to this desired softening is assigned to the particle (choosing only from those softening values explicitly input as a SofteningComovingTypeX parameter). This option is primarily useful for zoon simulations, where one may for example lump all boundary dark matter particles together into type 2 or 3, but yet provide a set of softening types over which they are automatically distributed according to their mass. If both ADAPTIVE_HYDRO_SOFTENING and MULTIPLE_NODE_SOFTENING are set, the softening types considered for assignment exclude softening type 0. Note: particles that accrete matter (BHs or sinks) get their softening updated if needed.
NSOFTTYPES=6: This can be changed to modify the number of available softening types. These must be explicitly input as SofteningComovingTypeX parameters, and so the value of NSOFTTYPES must match the number of these entries in the parameter file.
NSOFTTYPES_HYDRO=64: This is only relevant if adaptive hydro softening is enabled and can be set to override the default value of 64 for the length of the logarithmically spaced softening table. The sum of NSOFTTYPES and NSOFTTYPES_HYDRO may not exceed 254 (this is checked). Default value generally recommended.
DO_NOT_RANDOMIZE_DOMAINCENTER: Disable mitigations against time-correlated gravity force errors which minimize the center-of-mass drift of isolated objects.
CELL_CENTER_GRAVITY: Uses geometric centers (instead of mesh-generating points) to calculate gravity of cells, only possible with HIERARCHICAL_GRAVITY.
ALLOW_DIRECT_SUMMATION: Performs direct summation instead of tree-based gravity if number of active particles < DIRECT_SUMMATION_THRESHOLD (= 3000 unless specified differently)
DIRECT_SUMMATION_THRESHOLD=N: Overrides maximum number of active particles for which direct summation is performed instead of a tree based calculation.
EXACT_GRAVITY_FOR_PARTICLE_TYPE=N: Enables direct summation, i.e. O(N^2), gravity calculation for the given particle type.

Particle-mesh (PM) gravity 

PMGRID=512: Dimension of particle-mesh grid covering the domain. This enables the TreePM method, i.e. the long-range force is computed with a PM-algorithm, and the short range force with the tree. The parameter has to be set to the size of the mesh that should be used, e.g. 64, 128, 256, etc. The mesh dimensions need not necessarily be a power of two, but the FFT is fastest for such a choice. Note: If the simulation is not in a periodic box, then a FFT method for vacuum boundaries is employed, using a mesh with dimension twice that specified by PMGRID. Recommended value: for periodic boxes, twice the cube root of the number of dark matter particles, i.e. 512 for a \(256^3\) simulation.
ASMTH=1.25: This factor expressed the adopted force split scale in the TreePM approach in units of the grid cell size. Setting this value overrides the default value of 1.25, in mesh-cells, which defines the long-range/short-range force split.
RCUT=6.0: This determines the maximum radius, in units of the force split scale, out to which the tree calculation in TreePM mode considers tree nodes. If a tree node is more distant, the corresponding branch is discarded. The default value is 4.5, given in mesh-cells.
PLACEHIGHRESREGION=2: If this option is set (will only work together with PMGRID), then the long range force is computed in two stages: One Fourier-grid is used to cover the whole simulation volume, allowing the computation of the large-scale force. A second Fourier mesh is placed on the region occupied by “high-resolution” particles, allowing the computation of an intermediate-scale force. Finally, the force on very small scales is computed by the tree. This procedure can be useful for “zoom-simulations”, where the majority of particles (the high-res particles) are occupying only a small fraction of the volume. To activate this option, the parameter needs to be set to an integer that encodes the particle types that make up the high-res particles in the form of a bit mask. For example, if types 0, 1, and 4 are the high-res particles, then the parameter should be set to PLACEHIGHRESREGION=1+2+16, i.e. to the sum \(2^0+2^1+2^4\). The spatial region covered by the high-res grid is determined automatically from the initial conditions. The region is recalculated if one of the selected particles is falling outside of the high-resolution region. Note: If a periodic box is used, the high-res zone is not allowed to intersect the box boundaries.
ENLARGEREGION=1.1: This is only relevant when PLACEHIGHRESREGION is activated. The size of the high resolution box will be automatically determined as the minimum size required to contain the selected particle type(s), in a “shrink-wrap” fashion. This region is be expanded on the fly, as needed. However, in order to prevent a situation where this size needs to be enlarged frquently, such as when the particle set is (slowly) expanding, the minimum size is multiplied by the factor ENLARGEREGION (if defined). Then even if the set is expanding, this will only rarely trigger a recalculation of the high resolution mesh geometry, which is in general also associated with a change of the force split scale. Recommended value: between 1.1 and 1.4.
GRIDBOOST=1: Normally, if PLACEHIGHRESREGION is enabled, the code will try to offer an effective grid size for the high-resolution patch that is equivalent to PMGRID. Because zero-padding has to be used for the high-res inset, this gives a total mesh twice as large, which corresponds to GRIDBOOST=2. This value can here be increased by hand, to e.g. 4 or 8, to increase the resolution of the high-res PM grid. The total mesh size used for the high-resolution FFTs is given by GRIDBOOST*PMGRID.
ONLY_PM: This option disables the short-range gravitational force calculation by the tree, i.e. the gravitational force only consists of the PM force. Not recommended.
PM_ZOOM_OPTIMIZED: This option enables a different communication algorithm in the PM calculations (the one of Gadget3) which works well independent of the data layout, in particular it can cope well with highly clustered particle distributions that occupy only a small subset of the total simulated volume. However, this method is a bit slower than the default approach (used when the option is disabled), which is best matched for homogenously sampled periodic boxes. Recommended only for large zoom runs.
FFT_COLUMN_BASED: When this is enabled, the FFT calculations are not parallelized in terms of a slab-decomposition but rather through a column based approach. This scales to larger number of MPI ranks but is slower in absolute terms as twice as many transpose operations neeed to be performed. It is hence only worthwhile to use this option for very large number of MPI ranks that exceed the 1D mesh dimension. Recommended only for very large runs (MPI task number exceeding 4096 or 8192?).
CHUNKING: This will calculate the gravity forces in interleaved blocks and is a pure optimization option that can help to improve imbalances in the communication patterns in case multiple iterations in a force calculation are needed due to an insufficient buffer size. Always recommended.

External gravity 

By default, there is no “external” (i.e. fixed) gravitational potential, although one can be added.

EXTERNALGRAVITY: Master switch, enable an external potential. In this case, choose one of the options below.
EXTERNALGY=0.0: Constant external gravity in the y-direction
STATICNFW: Static gravitational Navarro-Frenk-White (NFW) potential.
NFW_C=12: If STATICNFW, also specify: concentration parameter of NFW potential.
NFW_M200=100.0: If STATICNFW, also specify: mass causing the NFW potential.
NFW_Eps=0.01: If STATICNFW, also specify: softening of NFW potential.
NFW_DARKFRACTION=0.87: If STATICNFW, also specify: fraction of dark matter in NFW potential. The potential will be reduced by this factor (with the idea being that the complement is respresented by gas mass included explicitly in the simulation).
STATICISO: Static gravitational isothermal sphere potential.
ISO_M200=100.0: If STATICISO, also specify: mass causing the isothermal sphere potential.
ISO_R200=160.0: If STATICISO, also specify: radius of the isothermal sphere potential.
ISO_Eps=0.1: If STATICISO, also specify: softening of isothermal sphere potential.
ISO_FRACTION=0.9: If STATICISO, also specify: fraction in dark matter in isothermal sphere potential. Potential will be reduced by this factor.
STATICHQ: Static gravitational Hernquist potential.
HQ_M200=186.015773: If STATICHQ, also specify: mass causing the Hernquist potential.
HQ_C=10.0: If STATICHQ, also specify: concentration parameter of Hernquist potential.
HQ_DARKFRACTION=0.9: If STATICHQ, also specify: fraction in dark matter in Hernquist potential. Potential will be reduced by this factor.

Voronoi Mesh 

Mesh motion & regularization 

VORONOI_STATIC_MESH: Assumes the mesh to be static, i.e. to not change with time. The vertex velocities of all mesh-generating points is set to zero and domain decomposition is disabled.
VORONOI_STATIC_MESH_DO_DOMAIN_DECOMPOSITION: Enables domain decomposition together with VORONOI_STATIC_MESH (which is otherwise then disabled), in case non-gas particle types exist and the use of domain decompotions is desired. Note that on one hand it may be advantageous in case the non-gas particles mix well or cluster strongly, but on the other hand the mesh construction that follows the domain decomposition is slow for a static mesh, so whether or not using this new flag is overall advantageous depends on the problem.

Hydrodynamics 

Riemann solver 

By default, an iterative, exact (hydrodynamics) Riemann solver is used. If one of the flags below is active, this is changed. Only one Riemann solver can be active.

VARIABLE_GAMMA: todo.
RIEMANN_HLL: todo.
RIEMANN_HLLC: Enable the HLLC approximate Riemann solver.
RIEMANN_ROSUNOV: todo.
RIEMANN_HLLD: Enable the HLLD approximate Riemann solver (required for MHD).
RIEMANN_GAMMA: todo.

Slope Limiting and Reconstruction 

TVD_SLOPE_LIMITER: Apply a componentwise total variation dimishing slope limiter. Use one of the TVD_SLOPE_LIMITER_* options to select the limiter.
TVD_SLOPE_LIMITER_ALBADA: Chooses the van Albada limiter. Use together with option TVD_SLOPE_LIMITER.
TVD_SLOPE_LIMITER_MINBEE: Chooses the minbee limiter. Use together with option TVD_SLOPE_LIMITER.
TVD_SLOPE_LIMITER_SUPERBEE: Chooses the superbee limiter. Use together with option TVD_SLOPE_LIMITER.
TVD_SLOPE_LIMITER_VANLEER: Chooses the van Leer limiter. Use together with option TVD_SLOPE_LIMITER.
TVD_SLOPE_LIMITER_MINMOD: todo
TVD_SLOPE_LIMITER_MC: todo
DISABLE_TIME_EXTRAPOLATION: todo
DISABLE_SPATIAL_EXTRAPOLATION: todo
NO_SCALAR_GRADIENTS: todo
GRADIENT_LIMITER_DUFFELL: todo

Time integration 

TREE_BASED_TIMESTEPS: Non-local timestep criterion (which takes the ‘signal speed’ of hydrodynamical waves arriving from any point into account). This is the fiducial behavior for cosmological simulations.
FORCE_EQUAL_TIMESTEPS: Variable but global timestep. Here the tightest timestep criterion evaluated for any of the particles determines the timestep of all particles. This is typically only for test problems.
PM_TIMESTEP_BASED_ON_TYPES=2+4: Particle types that should be considered in setting the PM timestep.
NO_PMFORCE_IN_SHORT_RANGE_TIMESTEP: PM force is not included in short-range timestep criterion.
ENLARGE_DYNAMIC_RANGE_IN_TIME: This extends the dynamic range of the integer timeline from 32 to 64 bits.

Cooling & star formation 

USE_SFR: Master switch to enable star formation following the Springel & Hernquist (2003) model.
MODIFIED_EOS: The effective equation of state is modified close to the SF density threshold to make it harder.
SFR_KEEP_CELLS: Forces derefinement of cells that formed stars?
SLOW_RELAX_TO_EOS: Temperature of star-forming gas is relaxed slowly to the eEOS only if SLOW_RELAX_TO_EOS is defined.
STEEPER_SFR_FOR_STARBURST: Changes the scaling of the star formation rate with density to \(\rm{SFR} \propto \rho^{\alpha}\) for densities greater than the turnover point for the eEOS (where run-away sets in). Here, \(\alpha\) corresponds to the StarburstPowerLawIndex parameter which must then be set. See Star formation model for documentation.

Input/Output options 

HAVE_HDF5: If this is set, the code will be compiled with support for input and output in the HDF5 format. You need to have the HDF5 libraries and headers installed on your computer for this option to work. The HDF5 format can then be selected as format “3” in Arepo’s parameterfile. Always recommended.
REDUCE_FLUSH: If enabled files and stdout are only flushed after a certain time defined in the parameter file (standard behavior: everything is flashed most times something is written to it).
EVALPOTENTIAL: When this option is set, the code will compute the gravitational potential energy each time a global statistics is computed. This can be useful for testing global energy conservation.
TOLERATE_WRITE_ERROR: Tries to store files for which things go wrong in an alternative output directory, as specified by the AlternativeOutputDir parameter.
SUBBOX_SNAPSHOTS: Instructs the code to write additional “subbox”-type snapshots in addition to normal snapshots. Subbox snapshots are sub-volumes, and contain the full particle information within those sub-volumes. They are meant to be written very frequently, for high time frequency analysis or visualization. An arbitrary number of subboxes can be requested at once. Configuration is through a number of required parameters - see Subboxes for documentation.
READ_IN_ALL_IC_FIELDS: Tries to load all fields from IC, similar to what is usually done in restart mode 2. However, different from restart mode 2, the header is not adopted for simulation parameters, but the parameters in the parameter files are considered. For fields that are not present in the initial conditions but in the output, the field values will be initialized with zero. If this is not active, only the most fundamental fields are loaded.
MULTIPLE_RESTARTS: Keep several restart files instead of just last two copies.

Optional additional outputs 

OUTPUTPOTENTIAL: This will force the code to compute gravitational potentials for all particles each time a snapshot file is generated. These values are then included in the snapshot files. Note that the computation of the values of the potential costs additional time.
OUTPUTACCELERATION: This will include the physical acceleration of each particle in snapshot files.

Special Behavior 

Generally options in this category are recommended only for specialized simulations. They affect the physics, physical setup, or type of simulation.

ISOTHERM_EQS: This special option makes the gas behave like an isothermal gas with equation of state \(P = c_s^2 \rho\). The sound-speed \(c_s\) is set by the thermal energy per unit mass in the intial conditions, i.e. \(c_s^2=u\). If the value for \(u\) is zero, then the initial gas temperature in the parameter file is used to define the sound speed according to \(c_s^2= k T/m_p\), where \(m_p\) is the proton mass.
LONGIDS: If this is set, the code assumes that particle-IDs are stored as 64-bit long integers. This is only really needed if you want to go beyond ~2 billion particles.
RUNNING_SAFETY_FILE: If used, run will not start in case a file ‘./running’ exists. This may be useful if the same job is submitted several times (to deal with long queue wait times), to prevent the job from actually starting several times. The file is automatically written at the beginning of the run, and automatically moved to ‘running_done’ if the run exited without any problems.
GENERATE_GAS_IN_ICS: This will split every input particle into a gas cell and a dark matter particle, with the mass ratio following the cosmic average mass and baryon fractions, Omega0 and OmegaBaryon, respectively, which are defined in the parameter file. Useful to read-in gravity-only initial conditions for cosmological simulations.

Single- or double-precision 

DOUBLEPRECISION=1: If this option is disabled, the code uses single precision variables for storing most data associated with particles and cells. If it is activated (or set to 1), then all particle/cell data is stored in double precision internally. Alternatively, setting DOUBLEPRECISION=2 enables a mixed mode where some of the more crucial data is kept in double precision internally, whereas the bulk is stored in single precision. Note that output files are nevertheless written by converting the values that are saved to single precision. Finally, the new option DOUBLEPRECISION=3 uses a MySingle datatype, which for this setting becomes float, otherwise it is treated like MyFloat.
DOUBLEPRECISION_FFTW: This enables the use of double precision variables for storing the fields that are processed with the Fourier transforms. If this is set, the code will also then use the double-precision version of FTTW for the PM gravity calculations.
OUTPUT_IN_DOUBLEPRECISION: When enabled, the code stores snapshot files (as well as group catalogues) in double precision. This applies to all floating-point values, which then take 8 bytes each instead of 4.
INPUT_IN_DOUBLEPRECISION: When this is enabled, initial conditions files or snapshot files used as code input are read as double precision variables. (Normally, an error message should be produced if the input data is instead stored in single precision.) This should be deprecitated in the future for HDF5 ICs from which the precision of all input datasets is determined automatically.
AUTO_SWAP_ENDIAN_READIC: Enables automatic swapping of the endianness when reading ICs in the file formats 1 and 2. For file format 3, HDF5 is supporting this functionality as well. This should be deprecitated in the future for HDF5 ICs.

Technical Code Behavior 

These options are largely numerical, related to performance, optimizations, etc.

NOSTOP_WHEN_BELOW_MINTIMESTEP: If this is activated, the code will not terminate when the timestep falls below the value of MinSizeTimestep specified in the parameterfile. This is useful for runs where one wants to enforce a constant timestep for all particles. This can be done by activating this option, and by setting MinSizeTimestep and MaxSizeTimestep to an equal value.
MPI_HYPERCUBE_ALLGATHERV: Implements a workaround for some MPI-libraries that use quite a lot of internal storage for MPI_Allgatherv.
OPTIMIZE_MEMORY_USAGE: Optimize for memory, not for speed, related to the Voronoi mesh. Reduces sizeof(point) but requires more bit shifts / operations for integer mapping. Note: this is dangerous for high dynamic range simulations with mixed precision, since some position variables are singles instead of doubles.
NUM_THREADS: Enable OpenMP (per-node threading) parallelization. Caution should be taken to understand if, and how, portions of the code you are running have been parallelized in this manner.
IMPOSE_PINNING: Enforce pinning of MPI tasks to cores. Note that the MPI library typically does this automatically.
IMPOSE_PINNING_OVERRIDE_MODE: Override MPI pinning, if present.
GPU: Main switch to activate GPU support. Implements the GPU manager that handles GPU memory and execution. Must be enabled along with either GPU_CUDA or GPU_HIP.
GPU_CUDA: Use CUDA for Nvidia GPUs. Requires GPU to be enabled and CUDA toolkit to be installed.
GPU_HIP: Use HIP for AMD GPUs. Requires GPU to be enabled and ROCm/HIP to be installed.
GPU_DEBUG_ON_CPU: Run GPU code on CPU instead of GPU. Useful only for debugging GPU kernels without actual GPU hardware.
GPU_DEBUG: Add additional runtime checks and validation for GPU code execution. Helps identify memory issues and kernel launch problems.
DISABLE_GPU_MEMORY_MANAGER: Bypass the stack-like device memory manager: every gpumalloc/gpufree call becomes a direct backend malloc/free, and GpuMaxMemSize is ignored. This gives device-memory debugging tools (e.g. compute-sanitizer) one real allocation per block so they can detect out-of-bounds accesses. Analogous to DISABLE_MEMORY_MANAGER for the host memory manager. Never use in production runs.
GPU_FINITE_VOLUME_GRADIENTS: Calculate finite volume gradients using GPU acceleration with Least Squares Fit (LSF) method.
VTUNE_INSTRUMENT: Enables support for intel vtune. Sections marked with TIMER_START/STOP will be shown in the timeline.

On the fly and post-processed analysis 

Power spectra 

POWERSPECTRUM_ON_THE_FLY: Calculate matter power spectra (normal and folded power spectra for all particles and individually for each particle type) written to separate files whenever a snapshot is written.
POWERSPECTRUM_IN_POSTPROCESSING: Create a slimmed-down version of the code which can only be used with RestartFlag 11 (see Getting Started), i.e. to calculate power spectra for existing snapshots.

Friends of friends 

FOF: Master switch to enable the friends-of-friends group finder code. This will then usually be applied automatically before snapshot files are written (unless disabled selectively for certain output dumps).
FOF_PRIMARY_LINK_TYPES=2: This option selects the particle types that are processed by the friends-of-friends linking algorithm. A default linking length of 0.2 is assumed for this particle type unless specified otherwise.
FOF_SECONDARY_LINK_TYPES=1+16+32: With this option, FOF groups can be augmented by particles/cells of other particle types that they “enclose”. To this end, for each particle among the types selected by the bit mask specifed with FOF_SECONDARY_LINK_TYPES, the nearest among FOF_PRIMARY_LINK_TYPES is found and then the particle is attached to whatever group this particle is in.
FOF_SECONDARY_LINK_TARGET_TYPES=1: The motivation why this exists is unclear to me at present… A new option to make the secondary linking work better in zoom runs (after the FOF groups have been found, the tree is newly constructed for all the secondary link targets). This should normally be set to all dark matter particle types. If not set, it defaults to FOF_PRIMARY_LINK_TYPES, which reproduces the old behavior.
FOF_GROUP_MIN_LEN=32: This sets the minimum total number of particles/cells (primary plus secondary) that a group must have before it is stored in the group catalogue. The default value for this is 32.
FOF_LINKLENGTH=0.2: This can be set to override the default value of the FOF linking length in units of the mean particle spacing of the primary link type.
FOF_FUZZ_SORT_BY_NEAREST_GROUP=0: DESCRIPTION NEEDED.
FOF_STOREIDS: Normally, the snapshots produced with a FOF group catalogue are stored in group order, such that the particle set making up a group can be inferred as a contiguous block of particles in the snapsot file, making it redundant to separately store the IDs of the particles making up a group in the group catalogue. By activating this option, one can nevertheless force to create the corresponding lists of IDs as part of the group catalogue output.
ADD_GROUP_PROPERTIES: This can be used to calculate additional properties for an already existing group catalogue. The snapshot is loaded, the substructure calculations are made, and the new properties are then added as additional datasets to the HDF5 group catalog files. The run then exits.
USE_AREPO_FOF_WITH_GADGET_FIX: Recommened when applying Arepo’s FOF group finder in post-processing mode on prexisting Gadget snapshot files that contain gas. Then the SPH smoothing lengths stored in the Gadget file can be used as initial guesses for the secondary linking stage.

Subfind 

SUBFIND: When enabled, this automatically runs the Subfind analysis of all FOF groups after they have been found. This snapshot files are brought into subhalo order within each group.
SAVE_HSML_IN_SNAPSHOT: When activated, this will store the hsml-values used for estimating total matter density around every point and the corresonding densities in the snapshot files associated with a run of Subfind.
SUBFIND_MEASURE_H2MASS: This is a special measuremenat option for the mass in molecular hydrogen found in each subhalo.
SUBFIND_CALC_MORE: Additional calculations are carried out in the Subfind algorithm, which may be expensive. (i) The velocity dispersion in the local density estimate. (ii) The DM density around every particle is stored in the snapshot if this is set together with SAVE_HSML_IN_SNAPSHOT.
SUBFIND_EXTENDED_PROPERTIES: Additional calculations are carried out, which may be expensive. (i) Further quantities related to the angular momentum in different components. (ii) The kinetic, thermal and potential binding energies for SO halos.

Visualization 

VORONOI_MESHOUTPUT: todo.
VORONOI_IMAGES_FOREACHSNAPSHOT: todo.
VORONOI_FREQUENT_IMAGES: Can be used to create images with frequency TimeBetweenImages set in the parameter file, independent of snapshot files. Note that this is only guaranteed to work correctly with a constant and equal image spacing if TimeBetweenImages >= MaxSizeTimestep.
VORONOI_FIELD_DUMP_PIXELS_X=1024: todo.
VORONOI_FIELD_DUMP_PIXELS_Y=1024: todo.
VORONOI_VELOCITY_FIELD_2D: todo.
VORONOI_FIELD_COMPENSATE_VX=0.0: todo.
VORONOI_FIELD_COMPENSATE_VY=0.0: todo.
VORONOI_NEW_IMAGE: todo.
VORONOI_PROJ_TEMP: todo.
VORONOI_PROJ: todo.
VORONOI_MULTIPLE_PROJECTIONS: Carries out two projections when a projection is called for, swapping axisY and axisZ (e.g. faceon and edgeon). The image filenames have the axis appended, for example “density_proj_x0_y1” and “density_proj_x0_y2”. This option is provided for convenience, especially when images are made “on-the-fly”. Note that, at present, slices are unaffected.
VORONOI_NOGRADS: todo.