userguide/config-options.md

+# Code Configuration
+
+
+Basic operation mode
+============================
+
+The default running mode (without any of the flags active) is 3d with 6 particle types; 
+type 0 is always gas; types >0 are only gravitationally interacting.
+
+
+**NTYPES=6**
+
+The number of particle types used. Minimum: 2.
+
+-----
+
+**TWODIMS**
+
+Simulation in 2d. Z coordinates and velocities are set to zero after reading 
+in initial conditions.
+
+-----
+
+**ONEDIMS**
+
+Simulation in 1d. Y and Z coordinates and velocities are set to zero after 
+reading in initial conditions.
+
+-----
+
+**ONEDIMS_SPHERICAL**
+
+Spherically symmetric 1d simulation. Use together with ``ONEDIMS``.
+The first dimension is used as the radial coordinate.
+
+-----
+
+Computational box
+================================
+
+The default running mode (without any of the flags active) is a cubic box with 
+periodic boundary conditions
+
+**LONG_X=10.0**
+
+These options can be used to distort the 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.)
+
+-----
+
+**LONG_Y=2.0**
+
+Stretches the y extent of the computational box by a given factor.
+
+-----
+
+**LONG_Z=10.0**
+
+Stretches the z extent of the computational box by a given factor.
+
+-----
+
+**REFLECTIVE_X=1**
+
+Boundary conditions in x direction. 1: Reflective, 2: Inflow/Outflow; not set: periodic
+
+-----
+
+**REFLECTIVE_Y=1**
+
+Boundary conditions in y direction. 1: Reflective, 2: Inflow/Outflow; not set: periodic
+
+-----
+
+**REFLECTIVE_Z=1**
+
+Boundary conditions in z direction. 1: Reflective, 2: Inflow/Outflow; not set: periodic
+
+-----
+
+Hydrodynamics
+=============
+
+The default mode is: ``GAMMA=5/3`` ideal hydrodynamics
+
+**NOHYDRO**
+
+No hydrodynamics calculation. Note that simply not including any type 0 
+particles has the same effect.
+
+-----
+
+**GAMMA=1.4**
+
+Adiabatic index of gas. 5/3 if not set.
+
+-----
+
+**ISOTHERM_EQS**
+
+Isothermal gas. Code uses an isothermal Riemann-solver.
+
+-----
+
+**PASSIVE_SCALARS=3**
+
+Number of passive scalar fields advected with fluid (default: 0).
+
+-----
+
+**NO_SCALAR_GRADIENTS**
+
+Disables time and spatial extrapolation for passive scalar fields. Use only if
+you know why you are doing this.
+
+-----
+
+Magnetohydrodynamics
+====================
+
+By default, code only computes hydrodynamics. Note that for comparison of 
+MHD and hydrodynamics runs, it is sometimes useful to keep the MHD settings 
+active and to initialize the magnetic field to zero everywhere. The equations 
+of ideal MHD ensure that the magnetic field stays exactly zero throughout 
+the calculation.
+
+**MHD**
+
+Master switch for magnetohydrodynamics.
+
+-----
+
+**MHD_POWELL**
+
+Powell div(B) cleaning scheme for magnetohydrodynamics.
+
+-----
+
+**MHD_POWELL_LIMIT_TIMESTEP**
+
+Additional timestep constraint due to Powell cleaning scheme.
+
+-----
+
+**MHD_SEEDFIELD**
+
+Uniform magnetic seed field of specified orientation and strength set up after reading in IC.
+
+-----
+
+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.
+
+**RIEMANN_HLLC**
+
+HLLC approximate Riemann solver.
+
+-----
+
+**RIEMANN_HLLD**
+
+HLLD approximate Riemann solver (required for MHD).
+
+-----
+
+Mesh motion 
+==============================
+
+The default mode is a moving mesh.
+
+**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.
+
+-----
+
+**REGULARIZE_MESH_CM_DRIFT**
+
+Mesh regularization. Move mesh generating point towards center of mass to make 
+cells rounder.
+
+-----
+
+**REGULARIZE_MESH_CM_DRIFT_USE_SOUNDSPEED**
+
+Limits Mesh regularization speed by local sound speed.
+
+------
+
+**REGULARIZE_MESH_FACE_ANGLE**
+
+Uses maximum face angle as roundness criterion in mesh regularization.
+
+-----
+
+Refinement 
+===========================
+
+By default, there is no refinement and derefinement and unless set otherwise,
+the cirterion for refinement/derefenment is a target mass.
+
+**REFINEMENT_SPLIT_CELLS**
+
+Allows refinement.
+
+-----
+
+**REFINEMENT_MERGE_CELLS**
+
+Allows derefinement.
+
+-----
+
+**REFINEMENT_VOLUME_LIMIT**
+
+Limits the volume of cells and the maximum volume difference between 
+neighboring cels.
+
+-----
+
+**JEANS_REFINEMENT**
+
+Refinement criterion to ensure resolving the Jeans length of cells.
+
+-----
+
+**REFINEMENT_HIGH_RES_GAS**
+
+Limits the dynamical (de-)refinements of cells to cells which are either 
+already present in the ICs or are created with ``GENERATE_GAS_IN_ICS`` from 
+type 1 particles. This adds an additional integer quantity ``AllowRefinement`` 
+to PartType0 in the snapshots indicating if a gas cell is allowed to be 
+refined and if it is, how often this cell has already been split: if 0, no 
+splitting allowed. If odd (starting at 1), the cell was already present in the 
+ICs. If even (starting at 2), the cell was generated from a type 1 particle. 
+For values of 3 or more, ``floor((AllowRefinement-1)/2.0)`` gives the number 
+of times the cell was split.
+
+-----
+
+**NODEREFINE_BACKGROUND_GRID**
+
+The background grid will be prevented from derefining, when refinement is 
+used. In practice, when enabled this option requires an input parameter 
+``MeanVolume``. Derefinement is then disallowed during the 
+run for all cells with ``Volume > 0.1 * MeanVolume``.
+
+-----
+
+**OPTIMIZE_MESH_MEMORY_FOR_REFINEMENT**
+
+If activated some grid structures not needed for mesh refinement/derefinement 
+are freed before the function for refinement and derefinement is called. The 
+remaining mesh structures are freed after this step as usual.
+
+-----
+
+Non-standard phyiscs
+====================
+
+**COOLING**
+
+Simple primordial cooling routine.
+
+-----
+
+**ENFORCE_JEANS_STABILITY_OF_CELLS**
+
+This imposes an adaptive floor for the temperature.
+
+-----
+
+**USE_SFR**
+
+Star formation model, turning dense gas into collisionless partices. See
+Springel&Hernquist, (2003, MNRAS, 339, 289)
+
+-----
+
+**SFR_KEEP_CELLS**
+
+Do not distroy cell out of which a star has formed.
+
+-----
+
+Gravity 
+=================
+
+If nothing is active, no gravity included.
+
+**SELFGRAVITY**
+
+Gravitational intraction between simulation particles/cells.
+
+-----
+
+**HIERARCHICAL_GRAVITY**
+
+Uses hierarchical splitting of the time integration of the gravity.
+
+-----
+
+**CELL_CENTER_GRAVITY**
+
+Uses geometric centers (instead of mesh-generating points) to calculate 
+gravity of cells, only possible with ``HIERARCHICAL_GRAVITY``.
+
+-----
+
+**NO_GAS_SELFGRAVITY**
+
+Switches off gas self-gravity in tree.
+
+-----
+
+**GRAVITY_NOT_PERIODIC**
+
+Gravity is not treated periodically.
+
+-----
+
+**ALLOW_DIRECT_SUMMATION**
+
+Performes direct summation instead of tree-based gravity if number of active 
+particles < ``DIRECT_SUMMATION_THRESHOLD`` (= 3000 unless specified differently)
+
+-----
+
+**DIRECT_SUMMATION_THRESHOLD=1000**
+
+Overrides maximum number of active particles for which direct summation is 
+performed instead of tree based calculation.
+
+-----
+
+**EXACT_GRAVITY_FOR_PARTICLE_TYPE=4**
+
+N-squared fashion gravity for a small number of particles of the given type.
+
+-----
+
+**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.
+
+-----
+
+**RANDOMIZE_DOMAINCENTER**
+
+Random displacement to position of domain center; avoids correlated 
+force-errors, important mainly for isolated systems (which otherwise might 
+start to drift in some direction).
+
+-----
+
+TreePM 
+==============
+
+If no switch is active: no Particle-Mesh calculation.
+
+**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. 256, 512, 1024 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``. Should not be used with ``PMGRID<256``
+because if this is active, the tree-force will be calculated assuming 
+non-periodic boundary conditions. This approximation is only valid if the 
+range of the tree calculation is small compared to the box size. 
+
+-----
+
+**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.
+
+-----
+
+**PM_ZOOM_OPTIMIZED**
+
+This option enables a different communication algorithm in the PM calculations
+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.
+
+-----
+
+**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. 
+
+-----
+
+**GRIDBOOST=2**
+
+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``.
+
+
+-----
+
+**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 need 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.
+
+-----
+
+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. Several particle types can be mapped to the same 
+softening if desired.
+
+**NSOFTTYPES=4**
+
+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.
+
+-----
+
+**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).
+
+-----
+
+**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 
+(black holes or sinks) get their softening updated if needed.
+
+-----
+
+**ADAPTIVE_HYDRO_SOFTENING**
+
+When this is enabled, the gravitational softening lengths of hydro 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.
+
+-----
+
+**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).
+
+-----
+
+External gravity
+================
+
+By default, there is no external potential.
+
+**EXTERNALGRAVITY**
+
+Master switch for external potential.
+
+-----
+
+**EXTERNALGY=0.0**
+
+Constant external gravity in y direction
+
+-----
+
+NFW Potential
+--------------------
+
+**STATICNFW**
+
+Static gravitational Navarro-Frenk-White (NFW) potential.
+
+-----
+
+**NFW_C=12**
+
+Concentration parameter of NFW potential.
+
+-----
+
+**NFW_M200=100.0**
+
+Mass causing the NFW potential.
+
+-----
+
+**NFW_Eps=0.01**
+
+Softening of NFW potential.
+
+-----
+
+**NFW_DARKFRACTION=0.87**
+
+Fraction in dark matter in NFW potential. Potential will be reduced by this
+factor.
+
+-----
+
+Isothermal Sphere
+----------------------------------
+
+**STATICISO**
+
+Static gravitational isothermal sphere potential.
+
+-----
+
+**ISO_M200=100.0**
+
+Mass causing the isothermal sphere potential.
+
+-----
+
+**ISO_R200=160.0**
+
+Radius of the isothermal sphere potential.
+
+-----
+
+**ISO_Eps=0.1**
+
+Softening of isothermal sphere potential.
+
+-----
+
+**ISO_FRACTION=0.9**
+
+Fraction in dark matter in isothermal sphere potential. Potential will be 
+reduced by this factor.
+
+-----
+
+Hernquist Potential
+--------------------------
+
+**STATICHQ**
+
+Static gravitational Hernquist potential.
+
+-----
+
+**HQ_M200=186.015773**
+
+Mass causing the Hernquist potential.
+
+-----
+
+**HQ_C=10.0**
+
+Concentration parameter of Hernquist potential.
+
+-----
+
+**HQ_DARKFRACTION=0.9**
+
+Fraction in dark matter in Hernquist potential. Potential will be reduced by 
+this factor.
+
+-----
+
+Time integration
+========================
+
+**FORCE_EQUAL_TIMESTEPS**
+
+Variable but global timestep.
+
+-----
+
+**TREE_BASED_TIMESTEPS**
+
+Non-local timestep criterion (take 'signal speed' into account).
+
+-----
+
+**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 bit
+
+-----
+
+Message Passing Interface
+========================================
+
+**IMPOSE_PINNING**
+
+Enforce pinning of MPI tasks to cores if MPI does not do it.
+
+-----
+
+**IMPOSE_PINNING_OVERRIDE_MODE**
+
+Override MPI pinning, if present.
+
+-----
+
+Single/Double Precision 
+=======================
+
+**DOUBLEPRECISION=1**
+
+Mode of double precision: not set: single; 1: full double precision 
+2: mixed, 3: mixed, fewer single precisions; unless short of memory, use 1.
+
+-----
+
+**DOUBLEPRECISION_FFTW**
+
+FFTW calculation in double precision.
+
+-----
+
+**OUTPUT_IN_DOUBLEPRECISION**
+
+Snapshot files will be written in double precision.
+
+-----
+
+**INPUT_IN_DOUBLEPRECISION**
+
+Initial conditions are in double precision.
+
+-----
+
+**OUTPUT_COORDINATES_IN_DOUBLEPRECISION**
+
+Will always output coordinates in double precision.
+
+-----
+
+**NGB_TREE_DOUBLEPRECISION**
+
+If this is enabled, double precision is used for the neighbor node extension.
+
+-----
+
+Groupfinder
+========================================
+
+**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. 
+Sum(2^type) for the primary dark matter type.
+
+-----
+
+**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. sum(2^type) for the types 
+linked to nearest primaries.
+
+-----
+
+**FOF_SECONDARY_LINK_TARGET_TYPES= 2**
+
+An 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 behaviour.
+
+-----
+
+**FOF_GROUP_MIN_LEN=32**
+
+Minimum number of particles (primary+secondary) in one group (default is 32).
+
+-----
+
+**FOF_LINKLENGTH=0.16**
+
+Linkinglength for FoF in units of the mean inter-particle separation. 
+(default=0.2)
+
+-----
+
+**FOF_FUZZ_SORT_BY_NEAREST_GROUP=0**
+
+Sort fuzz particles by nearest group and generate offset table in catalog 
+(=1 writes nearest group number to snapshot).
+
+-----
+
+**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.
+
+-----
+
+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_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 sperical 
+overdensity halos.
+
+
+-----
+
+Special behaviour 
+============================
+
+**RUNNING_SAFETY_FILE**
+
+If file './running' exists, do not start the run.
+
+-----
+
+**MULTIPLE_RESTARTS**
+
+Keep restart files instead of just last two copies.
+
+-----
+
+**EXTENDED_GHOST_SEARCH**
+
+This extends the ghost search to the full 3x3 domain instead of the principal 
+domain.
+
+-----
+
+**DOUBLE_STENCIL**
+
+This will ensure that the boundary region of the local mesh is deep enough to 
+have a valid double stencil for all local cells.
+
+-----
+
+**TETRA_INDEX_IN_FACE**
+
+Adds an index to each entry of VF[] and DC[] to one of the tetrahedra that 
+share this edge.
+
+-----
+
+**NOSTOP_WHEN_BELOW_MINTIMESTEP**
+
+Simulation does not terminate when timestep drops below minimum timestep.
+
+-----
+
+**TIMESTEP_OUTPUT_LIMIT**
+
+Limits timesteps to write snaps on time for frequent outputs.
+
+-----
+
+**ALLOWEXTRAPARAMS**
+
+Tolerate extra parameters that are not used.
+
+-----
+
+**FIX_SPH_PARTICLES_AT_IDENTICAL_COORDINATES**
+
+This can be used to load SPH ICs that contain identical particle coordinates.
+
+-----
+
+**RECOMPUTE_POTENTIAL_IN_SNAPSHOT**
+
+Needed for postprocess option 18 that can be used to calculate potential 
+values for a snapshot.
+
+-----
+
+**ACTIVATE_MINIMUM_OPENING_ANGLE**
+
+This does not open tree nodes under the relative opening criterion any more 
+if their opening angle has dropped below a minimum angle.
+
+-----
+
+**USE_DIRECT_IO_FOR_RESTARTS**
+
+Try to use O_DIRECT for low-level read/write operations of restart files to 
+circumvent the linux kernel page caching.
+
+-----
+
+**HUGEPAGES**
+
+Use huge pages for memory allocation, through hugetlbfs library.
+
+-----
+
+**DETAILEDTIMINGS**
+
+Creates individual timings entries for primary/secondary kernels to diagnose 
+work-load balancing.
+
+-----
+
+**BITS_PER_DIMENSION=42**
+
+Bits per dimension used in Peano-Hilbert key. (default: 42)
+
+-----
+
+
+Input options 
+=============
+
+**COMBINETYPES**
+
+Reads in the IC file types 4+5 as type 3.
+
+-----
+
+**LOAD_TYPES=1+2+4+16+32**
+
+Load only specific types sum(2^type).
+
+-----
+
+**READ_COORDINATES_IN_DOUBLE**
+
+Read coordinates in double precision.
+
+-----
+
+**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.
+
+-----
+
+**OFFSET_FOR_NON_CONTIGUOUS_IDS**
+
+Determines offset of IDs on startup instead of using fixed offset.
+
+-----
+
+**GENERATE_GAS_IN_ICS**
+
+Generates gas from dark matter only ICs (using particle type 1 by default).
+
+-----
+
+**SPLIT_PARTICLE_TYPE=4+8**
+
+Overrides splitting particle type 1 in ``GENERATE_GAS_IN_ICS`` use sum(2^type).
+
+-----
+
+**SHIFT_BY_HALF_BOX**
+
+Shift all positions by half a box size after reading in.
+
+-----
+
+**NTYPES_ICS=6**
+
+Number of particle types in ICs, if not ``NTYPES``.
+
+-----
+
+**READ_MASS_AS_DENSITY_IN_INPUT**
+
+Reads the mass field in the IC as density.
+
+-----
+
+Special input options 
+=====================
+
+**IDS_OFFSET=1**
+
+Override offset for gas particles if created from DM.
+
+-----
+
+**READ_DM_AS_GAS**
+
+Reads in dark matter particles as gas cells.
+
+-----
+
+**TILE_ICS**
+
+Tile ICs by TileICsFactor (specified as paramter) in each dimension.
+
+-----
+
+Output fields 
+==========================
+
+Default output filds are: ``position``, ``velocity``, ``ID``, ``mass``, 
+``specific internal energy`` (gas only), ``density`` (gas only)
+
+**OUTPUT_TASK**
+
+Output of MPI task.
+
+-----
+
+**OUTPUT_TIMEBIN_HYDRO**
+
+Output of hydrodynamics time-bin.
+
+-----
+
+**OUTPUT_PRESSURE_GRADIENT**
+
+Output of pressure gradient.
+
+-----
+
+**OUTPUT_DENSITY_GRADIENT**
+
+Output of density gradient.
+
+-----
+
+**OUTPUT_VELOCITY_GRADIENT**
+
+Output of velocity gradient.
+
+-----
+
+**OUTPUT_BFIELD_GRADIENT**
+
+Output of magnetic field gradient.
+
+-----
+
+**OUTPUT_MESH_FACE_ANGLE**
+
+Output of maximum face angle of cells.
+
+-----
+
+**OUTPUT_VERTEX_VELOCITY**
+
+Output of velocity of mesh-generating point.
+
+-----
+
+**OUTPUT_VOLUME**
+
+Output of volume of cells; note that this can always be computat as both, density 
+and mass of cells are by default in output.
+
+-----
+
+**OUTPUT_CENTER_OF_MASS**
+
+Output of center of mass of cells (``Pos`` is position of mesh-generating point).
+
+-----
+
+**OUTPUT_SURFACE_AREA**
+
+Output of surface area of cells as well as the number of faces.
+
+-----
+
+**OUTPUT_PRESSURE**
+
+Output of pressure of gas.
+
+-----
+
+**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**
+
+Output of gravitational acceleration.
+
+-----
+
+**OUTPUTTIMESTEP**
+
+Output of timestep of particle.
+
+-----
+
+**OUTPUT_SOFTENINGS**
+
+Output of particle softenings.
+
+-----
+
+**OUTPUTGRAVINTERACTIONS**
+
+Output of gravitatational interactions (from the gravitational tree) of particles.
+
+-----
+
+**OUTPUTCOOLRATE**
+
+Output of cooling rate.
+
+-----
+
+**OUTPUT_DIVVEL**
+
+Output of velocity divergence.
+
+-----
+
+**OUTPUT_CURLVEL**
+
+Output of velocity curl.
+
+-----
+
+**OUTPUT_COOLHEAT**
+
+Output of actual energy loss/gain in cooling/heating routine.
+
+-----
+
+**OUTPUT_VORTICITY**
+
+Output of vorticity of gas.
+
+-----
+
+**OUTPUT_CSND**
+
+Output of sound speed. This field is only used for tree-based timesteps! 
+Calculate from hydro quantities in postprocessing if required for science 
+applications.
+
+-----
+
+Output options 
+==============
+
+**PROCESS_TIMES_OF_OUTPUTLIST**
+
+Goes through times of output list prior to starting the simulaiton to ensure 
+that outputs are written as close to the desired time as possible (as opposed 
+to at next possible time if this flag is not active).
+
+-----
+
+**REDUCE_FLUSH**
+
+If enabled files and stdout are only flushed after a certain time defined in 
+the parameter file (standard behaviour: everything is flashed most times 
+something is written to it).
+
+-----
+
+**OUTPUT_EVERY_STEP**
+
+Create snapshot on every (global) synchronization point, independent of 
+parameters choosen or output list.
+
+-----
+
+**OUTPUT_CPU_CSV**
+
+Output of a cpu.csv file on top of cpu.txt.
+
+-----
+
+**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.
+
+-----
+
+**HDF5_FILTERS**
+
+Activate snapshot compression and checksum for HDF5 output.
+
+-----
+
+**OUTPUT_XDMF**
+
+Writes an ``.xmf`` file for each snapshot, which can be read by visit 
+(with the hdf5 snapshot). Note: so far only working if the snapshot 
+is stored in one file.
+
+-----
+
+Testing and Debugging
+=============================
+
+**DEBUG**
+
+Enables core-dumps.
+
+-----
+
+**VERBOSE**
+
+Reports readjustments of buffer sizes.
+
+-----
+
+Re-gridding
+============================
+
+These opitions are auxiliary modes to prepare/convert/relax initial conditions
+and will not carry out a simulation.
+
+**MESHRELAX**
+
+This keeps the mass constant and only regularizes the mesh.
+
+-----
+
+**ADDBACKGROUNDGRID=16**
+
+Re-grid hydrodynamics quantities on a Oct-tree AMR grid. This does not perform 
+a simulation. This "converts" an SPH initial condition into a (moving) mesh 
+initial condition.