Types of simulations

There are a number of qualitatively different simulation set-ups that can be run with GADGET-4, which differ mostly in the type of boundary conditions employed, in the physics that is included, whether or not cosmological integrations with comoving coordinates are used, and in the selection of numerical algorithms. A schematic overview of a subset of these simulation types, concentrating on how gravity calculations are done, is given in the following table.

Overview of simulation types

Type of Simulation	Computational Method	Remarks
Newtonian space	Gravity: Tree, SPH (optional), vacuum boundary conditions	OmegaLambda should be set to zero
Periodic long box	No gravity, only SPH, periodic boundary conditions	SELFGRAVITY needs to be deactivated, LONG_X/Y/Z may be set to scale the dimensions of the box
Cosmological, physical coordinates	Gravity: Tree, SPH, vacuum boundaries	ComovingIntegrationOn set to zero
Cosmological, comoving coordinates	Gravity: Tree, SPH, vacuum boundaries	ComovingIntegrationOn set to one
Cosmological, comoving periodic box	Gravity: Tree with Ewald-correction, SPH, periodic boundaries	PERIODIC needs to be set
Cosmological, comoving coordinates, TreePM	Gravity: Tree with long range PM, SPH, vacuum boundaries	PMGRID needs to be set
Cosmological, comoving periodic box, TreePM	Gravity: Tree with long range PM, SPH, periodic boundaries	PERIODIC and PMGRID need to be set
Cosmological, comoving coordinates, TreePM, Zoom	Gravity: Tree with long-range and intermediate-range PM, SPH, vacuum boundaries	PMGRID and PLACEHIGHRESREGION need to be set
Cosmological, periodic comoving box, TreePM, Zoom	Gravity: Tree with long-range and intermediate-range PM, SPH, periodic boundaries	PERIODIC, PMGRID and PLACEHIGHRESREGION need to be set
Newtonian space, TreePM	Gravity: Tree with long-range PM, SPH, vacuum boundaries	PMGRID needs to be set

Cosmological Simulations

Cosmological integrations with comoving coordinates are selected via the parameter ComovingIntegrationOn in the parameterfile. Otherwise the simulations always assume ordinary Newtonian space. The use of self-gravity needs to be explicitly enabled through the SELFGRAVITY compile-time switch, otherwise only external gravitational fields and/or hydrodynamical processes are computed. Periodic boundary conditions and the various variants of the Tree, FMM, TreePM, and FMM-PM algorithms require compile-time switches to be set appropriately in the configuration file.

In particular, the TreePM algorithm is switched on by passing the desired mesh-size at compile time via the Config.sh file to the code. The relevant parameter is PMGRID, see below. Using an explicit force split, the long-range force is then (normally) computed with Fourier techniques, while the short-range force is calculated with the tree. Because the tree needs only be walked locally, a speed-up can arise, particularly for near to homogeneous particle distributions, but not only restricted to them. Both periodic and non-periodic boundary conditions are implemented for the TreePM and FMM-PM approaches. In the non-periodic case, the code will internally compute FFTs of size HRPMGRID. In order to accommodate the required zero-padding, only half that size is actually used to cover the high-res region. If HRPMGRID is not specified, a default value equal to PMGRID is used. For zoom-simulations, the code automatically places the refined mesh-layer on the high-resolution region. This is controlled with the PLACEHIGHRESREGION option.

Newtonian space

Periodic boxes need not necessarily be cubical in GADGET-4. They can be, if desired, stretched independently in any of the coordinate directions through LONG_X, LONG_Y and LONG_Z options. This works both for SPH simulations and also for simulations including self-gravity. In the latter case, one may also choose one direction to be non-periodic in self-gravity, realizing mixed boundary conditions that allow one to simulate infinitely extended two-dimensional systems.

Stretched boxes

This can now also be done with gravity and periodic boundaries in just two dimensions.

Two-dimensional simulations

It is also possible to run SPH simulations in two dimensions only, which is primarily provided for test-purposes. Note that the code is not really optimised for doing this; three coordinates are still stored for all particles in this case, and the computations are formally carried out as in the 3D case, except that all particles lie in one coordinate plane, i.e. either with equal x, y, or z-coordinates.