our mascot GIZMO Users Guide


Table of Contents

  1. Code Overview
  2. Feature (Physics Module) Set
  3. Code Use, Authorship, Citation, Sharing, & Development Rules
  4. Fluid (Hydro) Solvers
  5. Similarities & Differences from GADGET (+other codes)
  6. Compiling and Using the Code (a very brief tutorial)
  7. Config.sh (Setting compile-time options)
  8. The Parameterfile (Setting run-time parameters)
  9. Snapshot & Initial Condition Files
  10. Log Outputs & Diagnostic Files
  11. Test Problems
  12. Useful Additional Resources
  13. Disclaimer

1. Code Overview

Welcome to GIZMO.

The simulation code GIZMO is a flexible, massively-parallel, multi-method multi-physics code, with a wide range of different physics modules described in the “Feature Set” below. The code is descended from P-GADGET, itself descended from GADGET–2, and is fully compatible with GADGET analysis codes, initial conditions, and snapshots. GIZMO is not an acronym – it refers both to the code’s multi-purpose applications and to its historical relationship to GADGET.

The code is extensively documented. Please read the User Guide before asking questions of me or other users: most questions I get are already answered here.

The code is written in standard ANSI C, and should run on all parallel platforms that support MPI. The portability of the code has been confirmed on a large number of systems ranging from a laptop to >1 million threads on national super-computers.

The original reference for the numerical methods (the original fluid+gravity solver) is the paper here. It is recommended that you read it before using (or modifying) the code. For more details on GADGET, which underpins some aspects here, see the GADGET–2 methods paper or GADGET–2 user guide.

The un-official code website (showing examples, demonstrations, and giving an overview of the code) is here, but the actual public code is hosted on Bitbucket here. The development (non-public) code repository is also hosted on Bitbucket, but in a separate repository here.

GIZMO was written by me, Philip F. Hopkins (PFH), although the codebase builds on parts of Volker Springel’s GADGET code. If you find the code useful, please reference the numerical methods paper in all studies using simulations run with GIZMO. You should also reference the GADGET paper for the domain decomposition and N-body algorithms. Various modules have their own methods papers that should be cited if they are used; these are specified in the “Config.sh (Setting compile-time options)” section of this Guide.


2. Feature Set

The GIZMO code is a flexible, massively parallel, multi-purpose fluid dynamics + gravity code. Some (not all!) current physics include:

Examples of Different Simulations Run with GIZMO



GIZMO and its methods have been used in a large number of simulations and well over a hundred different papers (for a partial list of works which reference the original methods paper, click here). Some examples are shown above, including: (a) Cosmology: Large-volume dark matter+baryonic cosmological simulation (Davé et al. 2016). (b) Galaxy formation: with cooling, star formation, and stellar feedback (Wetzel et al. 2016). (c) Dust (aero)-dynamics: grains moving with Epstein drag and Lorentz forces in a super-sonically turbulent cloud (Lee et al. 2017). (d) Black holes: AGN accretion according to resolved gravitational capture, with feedback driving outflows from the inner disk (Hopkins et al. 2016). (e) Proto-stellar disks: magnetic jet formation in a symmetric disk around a sink particle, in non-ideal MHD (Raives 2016). (f) Elastic/solid-body dynamics: collision and compression of two rubber rings with elastic stresses and von Mises stress yields. (g) Plasma physics: the magneto-thermal instability in stratified plasma in the kinetic MHD limit (Hopkins 2017). (h) Magneto-hydrodynamics: the Orszag-Tang vortex as a test of strongly-magnetized turbulence (Hopkins & Raives 2016). (i) Star formation: star cluster formation (with individual stars) including stellar evolution, mass loss, radiation, and collisionless dynamics (Grudic et al. 2016). (j) Fluid dynamics: the non-linear Kelvin-Helmholtz instability (Hopkins 2015). (k) Multi-phase fluids in the ISM/CGM/IGM: ablation of a cometary cloud in a hot blastwave with anisotropic conduction and viscosity (Su et al. 2017). (l) Impact and multi-material simulations: giant impact (lunar formation) simulation, after impact, showing mixing of different materials using a Tillotson equation-of-state (Deng et al. 2017).

Code Scaling (Parallelism & Performance)

GIZMO uses a hybrid MPI+OpenMP parallelization strategy with a flexible domain decomposition and hierarchical adaptive timesteps, which enable it to scale efficiently on massively-parallel systems with problem sizes up to and beyond billions of resolution elements. In addition, we have implemented a large number of optimizations for different problems and physics, including e.g. rolling sub-routines and communication together, implementing problem-specific weighting schemes for domain decomposition, solving gravity with a hybrid tree-nested particle mesh approach, and adopting fully adaptive spatial and time resolution. Remarkably, despite the additional computational complexity, the new fluid solvers are actually faster in GIZMO than their SPH counterparts, owing both to the reduced neighbor number needed and to several custom optimizations.

Code scalings are always (highly) problem-and-resolution-dependent, so it is impossible to make completely general statements about how GIZMO scales. However, we can illustrate one example here. This shows the code scalings of GIZMO (in hybrid MPI+OpenMP mode) on full production-quality FIRE–2 cosmological simulations of galaxy formation, including the full physics of self-gravity, hydrodynamics, cooling, star formation, and stellar feedback (from Hopkins et al. 2017, arXiv:1702.06148).



Weak scaling (making the computational problem larger in size, but increasing the processor number proportionally) for a full cosmological box, populated with high resolution particles (fixed particle mass), run for a short fraction of the age of the Universe. We increase the cosmological volume from 2 to 10,000 cubic comoving Mpc. In these units, theoretically “ideal” weak scaling would be flat (dashed line shown). The weak scaling of GIZMO’s gravity+MHD algorithm is near-ideal (actually slightly better at intermediate volume, owing to fixed overheads and statistical homogeneity of the volume at larger sizes) to greater than a million threads (>250,000 CPUs and >16,000 independent nodes).



Strong scaling (CPU time on a fixed-size problem, increasing the processor number) for a "zoom-in” simulation of a Milky Way-mass galaxy (1e12 solar-mass halo, 1e11 solar-masses of stars in the galaxy) or a dwarf galaxy (1e10 solar-mass halo, 1e6 solar-masses in stars in the galaxy), each using 1.5e8 (150 million) baryonic particles in the galaxy, run to 25% of the present age of the Universe. In these units, “ideal” scaling is flat (dashed line), i.e. the problem would take same total CPU time (but shorter wall-clock time as it was spread over more processors). Our optimizations allow us to maintain near-ideal strong scaling on this problem up to ~16,000 cores per billion particles (~2000 for the specific resolution shown).



Test increasing the resolution instead of the problem size (specifically, increasing the particle number for the same Milky Way-mass galaxy, while increasing the processor number proportionally). Because the resolution increases, the time-step decreases (smaller structures are resolved), so the ideal weak scaling increases following the dashed line. The achieved scaling is very close to ideal, up to >16,000 cores for ~1 billion particles.

This is an extremely inhomogeneous, high-dynamic-range (hence computationally challenging) problem with very dense structures (e.g. star clusters) taking very small timesteps, while other regions (e.g. the inter-galactic medium) take large timesteps — more “uniform” problems (e.g. a pure dark-matter only cosmological simulation or pure-hydrodynamic turbulence simulation) can achieve even better scalings.

Important Notes on Public vs. Proprietary Code

The public version of the code has full functionality for the “core” physics in GIZMO, including almost all physics listed above. This is intended for a wide range of science applications, as well as learning, testing, and code methods studies. A few modules are not yet in the public code (i.e. they are only present in the development code, which is not public). Usually, this is because they are in active development and not yet de-bugged or tested at the level required for use “out of the box” – those modules will be made public as soon as this stage is reached. In some cases, however, it is because a module involves proprietary code developed by others with their own collaboration policies, which must be respected.

If you wish to contribute to the active code development or new physics, first read the use policies below, and then contact me directly. Access to the development code, however, DOES NOT imply permission to use any modules identified as proprietary either in the README or User Guide or Template_Config.sh file. These are modules being actively developed or used by their original authors to finish PhD theses and related projects. For example, the set of modules identified as part of the FIRE project are specifically developed by a collaboration which has its own authorship and collaboration policies (akin to many observational collaborations). I (PFH) do not have sole authority (even though I am a member of that collaboration) to grant anyone permission to use those parts of the code. If in doubt about whether use of a given module is permitted, ask.

Permission to access and use the development code, and any modules therein, is not transferrable. If you have postdocs/students/collaborators, they must request permission as well. You should not distribute copies of the development code. If you move on from the original project for which permissions were granted, you must re-request permissions. Permission is granted only for the originally specified use purposes.


3. Code Use, Authorship, Citation, Sharing, & Development Rules

Use, Authorship, & Citation Requirements

Citation Requirements:

Any paper using GIZMO should, at a minimum, cite the methods paper: Hopkins 2015 (“A New Class of Accurate, Mesh-Free Hydrodynamics Methods”). You should also cite the GADGET methods paper (Springel, 2005, MNRAS, 364, 1105) for the domain decomposition and N-body algorithms.

When using specific modules, the relevant code papers must also be cited: for example, papers using the MHD version of the code should cite the MHD methods paper: Hopkins & Raives 2015 (“Accurate, Meshless Methods for Magneto-Hydrodynamics”). Anisotropic diffusion operators (conduction, viscosity, radiation, cosmic rays, non-ideal MHD, passive scalar diffusion) should site Hopkins 2016 (“Anisotropic Diffusion in Mesh-Free Numerical Magnetohydrodynamics”). The citations required for the various code modules are all given in the “Config.sh (Setting Compile-Time Options)” section of this Guide, where the modules are described.

Remember, these modules were developed at great expense in both time and grant funding by their authors. Moreover readers may want to see more detailed numerical tests. So please, respect the citation requirements for all modules or your access to GIZMO will be revoked.

Authorship Requirements & Acceptable Use: Public Code:

If you are using the public version of GIZMO, there are no requirements for offering authorship of papers resulting from the code, only the citation requirements above.

Authorship Requirements & Acceptable Use: Private Code:

However, if you are using the development version of the code, it is important to remember that many of the modules are not provided for common use under any license whatsoever, regardless of whether or not you have permission to see/access them. It is not acceptable to use these modules for any work without the explicit consent of the authors of these modules. In general, access to the development version of the code is granted only on the condition that all users understand and agree to this, and will only use non-public modules which they have developed themselves or for which they have explicitly gotten the approval of the relevant authors.

In the Template_Config.sh file included in the code, many of the relevant modules specifically outline their guidelines – the relevant modules are specifically described as requiring consent of various authors or groups for their use. Again, these permissions are required to use these modules in any scientific work; access to the development code does not imply permission to use any modules labeled as proprietary anywhere in the User Guide, Template_Config.sh file, README, or source code.

But if you are unsure whether or not you have permission to use a module, please contact the developers. If you do not know whether or not you have permission to use some module, and it is not in the public code, the only safe assumption is that you do not have that permission.

For example, a few of the modules in GIZMO are specifically developed either as part of the FIRE project or are imported from GADGET–3, these have specific, known policies. Any projects using the FIRE stellar and/or black hole feedback physics routines must be agreed to by the entire FIRE core development team (PFH, D. Keres, C.A. Faucher-Giguere, and E. Quataert), and the FIRE collaboration has a requirement that project plans be approved by the collaboration before work begins, and that authorship be offered to all the team members and papers be distributed internally among the collaboration before they are made public. GADGET–3 has long-standing user policies set by its main developer, V. Springel; if you had permissions to use a specific module in GADGET–3, it is probably safe to assume that permission extends to its imported version in GIZMO, but if you are unsure and the code is not public, Volker is the person to contact. Other modules were developed by individual students/postdocs, as part of their ongoing research, and are proprietary - they can only be used with explicit permission from the developer.

Users who violate these policies will have their access to the private version[s] of GIZMO immediate revoked.

Code Sharing Guidelines

The public version of the code is free software, distributed under the GNU General Public License. This implies that you may freely distribute and copy the public code. You may also modify it as you wish, and distribute these modified versions as long as you indicate prominently any changes you made in the original code, and as long as you leave the copyright notices, and the no-warranty notice intact. Please read the General Public License for more details. Note that the authors retain their copyright on the code.

The private version of the code is closed and can only be shared with the explicit permission of the author (PFH). Any sub-section of the non-public code can only be shared, distributed, or incorporated into other work with the explicit permission of the author and the developer(s) of the relevant code (see the policies for acceptable use, above).

Permissions are not transferrable. If you are working with collaborators, postdocs, or students, they must request permission as well to access the development code, and their permissions are restricted to the same set of modules as the project lead. Do not distribute tarballs of the private code to others; they should be accessing the development repository if they are using the development code. Permission is granted on a project basis: if the scope of the project changes, or if new modules are needed, or any proprietary parts of the code requested, permission must be re-requested.

Users who violate these policies will have their access to the private version[s] of GIZMO immediate revoked.

Code Development & Extensions (New Physics and Code)

You are encouraged to study and modify this code (public or private)! Please, if you have ideas for interesting physics to add, add it! You are only asked to do a few things, to make your experience and that of everyone else much easier.

General Code-style Principles (Good Practices):


4. Fluid (Hydro) Solvers

GIZMO is unique in that it allows the user (at compile time) to choose the method with which they would like to solve the fluid (hydrodynamic, MHD, RHD) equations. This flexibility goes far deeper than in most codes. We’re not talking about the difference between e.g. different Riemann solvers or different ways of writing the SPH equation of motion, we’re talking about different methods for how the simulation volume is decomposed (e.g. the “grid” underlying everything).

Examples (Demonstrations, Animations, and Talks)