$$\renewcommand{\AA}{\text{Å}}$$

# 8.2.1. 2d simulations

You must use the dimension command to specify a 2d simulation. The default is 3d.

A 2d simulation box must be periodic in z as set by the boundary command. This is the default.

Simulation boxes in LAMMPS can be either orthogonal or triclinic in shape. Orthogonal boxes in 2d are a rectangle with 4 edges that are each perpendicular to either the x or y coordinate axes. Triclinic boxes in 2d are a parallelogram with opposite pairs of faces parallel to each other. LAMMPS supports two forms of triclinic boxes, restricted and general, which for 2d differ in how the box is oriented with respect to the xy coordinate axes. See the Howto triclinic for a detailed description of all 3 kinds of simulation boxes.

Here are examples of using the create_box command to define the simulation box for a 2d system.

# 2d orthogonal box using a block-style region
region mybox block -10 10 0 10 -0.5 0.5
create_box 1 mybox

# 2d restricted triclinic box using a prism-style region with only xy tilt
region mybox prism 0 10 0 10 -0.5 0.5 2.0 0.0 0.0
create_box 1 mybox

# 2d general triclinic box using a primitive cell for a 2d hex lattice
lattice       custom 1.0 a1 1.0 0.0 0.0 a2 0.5 0.86602540378 0.0 &
a3 0.0 0.0 1.0 basis 0.0 0.0 0.0 triclinic/general
create_box    1 NULL 0 5 0 5 -0.5 0.5


Note that for 2d orthogonal or restricted triclinic boxes, the box has a 3rd dimension which must straddle z = 0.0 in the z dimension. Typically the width of box in the z dimension should be narrow, e.g. -0.5 to 0.5, but that is not required. For a 2d general triclinic box, the a3 vector defined by the lattice command must be (0.0,0.0,1.0), which is its default value. Also the clo and chi arguments of the create_box command must be -0.5 and 0.5.

Here are examples of using the read_data command to define the simulation box for a 2d system via keywords in the header section of the data file. These are the same boxes as the examples for the create_box command

# 2d orthogonal box
-10 10   xlo xhi
0 10     ylo yhi
-0.5 0.5 zlo zhi       # this is the default, so no need to specify

# 2d restricted triclinic box with only xy tilt
-10 10   xlo xhi
0 10     ylo yhi
-0.5 0.5 zlo zhi       # this is the default, so no need to specify
2.0 0.0 0.0 xy xz yz

# 3d general triclinic box using a primitive cell for a 2d hex lattice
5 0 0              avec
2.5 4.3301270189 0 bvec
0 0 1              cvec           # this is the default, so no need to specify
0 0 -0.5           abc origin     # this is the default for 2d, so no need to specify


Note that for 2d orthogonal or restricted triclinic boxes, the box has a 3rd dimension specified by the zlo zhi values, which must straddle z = 0.0. Typically the width of box in the z dimension should be narrow, e.g. -0.5 to 0.5, but that is not required. For a 2d general triclinic box, the z component of avec and bvec must be zero, and cvec must be (0,0,1), which is the default. The z component of abc origin must also be -0.5, which is the default.

If using the create_atoms command to create atoms in the 2d simulation box, all the z coordinates of created atoms will be zero.

If using the read_data command to read in a data file of atom coordinates for a 2d system, the z coordinates of all atoms should be zero. A value within epsilon of zero is also allowed in case the data file was generated by another program with finite numeric precision, in which case the z coord for the atom will be set to zero.

Use the fix enforce2d command as the last fix defined in the input script. It ensures 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 added by other fixes will also be zeroed out.

Many of the example input scripts included in the examples directory are for 2d models.

Note

Some models in LAMMPS treat particles as finite-size spheres, as opposed to point particles. See the atom_style sphere and 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 command and the disc option for the fix nve/sphere, fix nvt/sphere, fix nph/sphere, fix npt/sphere commands.