# pair_style dsmc command

## Syntax

``` LAMMPS
pair_style dsmc max_cell_size seed weighting Tref Nrecompute Nsample
```

-   max_cell_size = global maximum cell size for DSMC interactions
    (distance units)
-   seed = random \# seed (positive integer)
-   weighting = macroparticle weighting
-   Tref = reference temperature (temperature units)
-   Nrecompute = re-compute v\*sigma_max every this many timesteps
    (timesteps)
-   Nsample = sample this many times in recomputing v\*sigma_max

## Examples

``` LAMMPS
pair_style dsmc 2.5 34387 10 1.0 100 20
pair_coeff * * 1.0
pair_coeff 1 1 1.0
```

## Description

Style *dsmc* computes collisions between pairs of particles for a direct
simulation Monte Carlo (DSMC) model following the exposition in
[(Bird)](Bird). Each collision resets the velocities of the two
particles involved. The number of pairwise collisions for each pair or
particle types and the length scale within which they occur are
determined by the parameters of the pair_style and pair_coeff commands.

Stochastic collisions are performed using the variable hard sphere (VHS)
approach, with the user-defined *max_cell_size* value used as the
maximum DSMC cell size, and reference cross-sections for collisions
given using the pair_coeff command.

There is no pairwise energy or virial contributions associated with this
pair style.

The following coefficient must be defined for each pair of atoms types
via the [pair_coeff](pair_coeff) command as in the examples above, or in
the data file or restart files read by the [read_data](read_data) or
[read_restart](read_restart) commands:

-   sigma (area units, i.e. distance-squared)

The global DSMC *max_cell_size* determines the maximum cell length used
in the DSMC calculation. A structured mesh is overlayed on the
simulation box such that an integer number of cells are created in each
direction for each processor\'s subdomain. Cell lengths are adjusted up
to the user-specified maximum cell size.

------------------------------------------------------------------------

To perform a DSMC simulation with LAMMPS, several additional options
should be set in your input script, though LAMMPS does not check for
these settings.

Since this pair style does not compute particle forces, you should use
the \"fix nve/noforce\" time integration fix for the DSMC particles,
e.g.

``` LAMMPS
fix 1 all nve/noforce
```

This pair style assumes that all particles will communicated to
neighboring processors every timestep as they move. This makes it
possible to perform all collisions between pairs of particles that are
on the same processor. To ensure this occurs, you should use these
commands:

``` LAMMPS
neighbor 0.0 bin
neigh_modify every 1 delay 0 check no
atom_modify sort 0 0.0
communicate single cutoff 0.0
```

These commands ensure that LAMMPS communicates particles to neighboring
processors every timestep and that no ghost atoms are created. The
output statistics for a simulation run should indicate there are no
ghost particles or neighbors.

In order to get correct DSMC collision statistics, users should specify
a Gaussian velocity distribution when populating the simulation domain.
Note that the default velocity distribution is uniform, which will not
give good DSMC collision rates. Specify \"dist gaussian\" when using the
[velocity](velocity) command as in the following:

``` LAMMPS
velocity all create 594.6 87287 loop geom dist gaussian
```

------------------------------------------------------------------------

## Mixing, shift, table, tail correction, restart, rRESPA info

This pair style does not support mixing. Thus, coefficients for all I,J
pairs must be specified explicitly.

This pair style does not support the [pair_modify](pair_modify) shift
option for the energy of the pair interaction.

The [pair_modify](pair_modify) table option is not relevant for this
pair style.

This pair style does not support the [pair_modify](pair_modify) tail
option for adding long-range tail corrections to energy and pressure.

This pair style writes its information to [binary restart
files](restart), so pair_style and pair_coeff commands do not need to be
specified in an input script that reads a restart file. Note that the
user-specified random number seed is stored in the restart file, so when
a simulation is restarted, each processor will re-initialize its random
number generator the same way it did initially. This means the random
forces will be random, but will not be the same as they would have been
if the original simulation had continued past the restart time.

This pair style can only be used via the *pair* keyword of the
[run_style respa](run_style) command. It does not support the *inner*,
*middle*, *outer* keywords.

------------------------------------------------------------------------

## Restrictions

This style is part of the MC package. It is only enabled if LAMMPS was
built with that package. See the [Build package](Build_package) page for
more info.

## Related commands

[pair_coeff](pair_coeff), [fix nve/noforce](fix_nve_noforce),
[neigh_modify](neigh_modify), [neighbor](neighbor),
[comm_modify](comm_modify)

## Default

none

------------------------------------------------------------------------

::: {#Bird}
**(Bird)** G. A. Bird, \"Molecular Gas Dynamics and the Direct
Simulation of Gas Flows\" (1994).
:::