# 2d simulations

Use the [dimension](dimension) command to specify a 2d simulation.

Make the simulation box periodic in z via the [boundary](boundary)
command. This is the default.

If using the [create_box](create_box) command to define a simulation
box, set the z dimensions narrow, but finite, so that the
[create_atoms](create_atoms) command will fill the 3d simulation box
with a single z plane of atoms - e.g.

``` LAMMPS
create box 1 -10 10 -10 10 -0.25 0.25
```

If using the [read data](read_data) command to read in a file of atom
coordinates, set the \"zlo zhi\" values to be finite but narrow, similar
to the create_box command settings just described. For each atom in the
file, assign a z coordinate so it falls inside the z-boundaries of the
box - e.g. 0.0.

Use the [fix enforce2d](fix_enforce2d) command as the last defined fix
to ensure that the z-components of velocities and forces are zeroed out
every timestep. The reason to make it the last fix is so that any forces
induced by other fixes will be zeroed out.

Many of the example input scripts included in the LAMMPS distribution
are for 2d models.

:::: note
::: title
Note
:::

Some models in LAMMPS treat particles as finite-size spheres, as opposed
to point particles. See the [atom_style sphere](atom_style) and [fix
nve/sphere](fix_nve_sphere) commands for details. By default, for 2d
simulations, such particles will still be modeled as 3d spheres, not 2d
discs (circles), meaning their moment of inertia will be that of a
sphere. If you wish to model them as 2d discs, see the [set
density/disc](set) command and the *disc* option for the [fix
nve/sphere](fix_nve_sphere), [fix nvt/sphere](fix_nvt_sphere), [fix
nph/sphere](fix_nph_sphere), [fix npt/sphere](fix_npt_sphere) commands.
::::