# pair_style lj/expand command

Accelerator Variants: *lj/expand/gpu*, *lj/expand/kk*, *lj/expand/omp*

# pair_style lj/expand/coul/long command

Accelerator Variants: *lj/expand/coul/long/gpu*,
*lj/expand/coul/long/kk*

## Syntax

``` LAMMPS
pair_style lj/expand cutoff
```

-   cutoff = global cutoff for lj/expand interactions (distance units)

## Examples

``` LAMMPS
pair_style lj/expand 2.5
pair_coeff * * 1.0 1.0 0.5
pair_coeff 1 1 1.0 1.0 -0.2 2.0

pair_style lj/expand/coul/long 2.5
pair_style lj/expand/coul/long 2.5 4.0
pair_coeff * * 1.0 1.0 0.5
pair_coeff 1 1 1.0 1.0 -0.2 3.0
```

## Description

Style *lj/expand* computes a LJ interaction with a distance shifted by
delta which can be useful when particles are of different sizes, since
it is different that using different sigma values in a standard LJ
formula:

$$E = 4 \epsilon \left[ \left(\frac{\sigma}{r - \Delta}\right)^{12} -
  \left(\frac{\sigma}{r - \Delta}\right)^6 \right]
  \qquad r < r_c + \Delta$$

$r_c$ is the cutoff which does not include the $\Delta$ distance. I.e.
the actual force cutoff is the sum of $r_c +
\Delta$.

For all of the *lj/expand* pair styles, the following coefficients 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, or by mixing as described below:

-   $\epsilon$ (energy units)
-   $\sigma$ (distance units)
-   $\Delta$ (distance units)
-   cutoff (distance units)

The $\Delta$ values can be positive or negative. The last coefficient is
optional. If not specified, the global LJ cutoff is used.

For *lj/expand/coul/long* only the LJ cutoff can be specified since a
Coulombic cutoff cannot be specified for an individual I,J type pair.
All type pairs use the same global Coulombic cutoff specified in the
pair_style command.

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

Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the [Accelerator packages](Speed_packages)
page. The accelerated styles take the same arguments and should produce
the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and
OPT packages, respectively. They are only enabled if LAMMPS was built
with those packages. See the [Build package](Build_package) page for
more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the [-suffix command-line
switch](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_packages) page for more
instructions on how to use the accelerated styles effectively.

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

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

For atom type pairs I,J and I != J, the epsilon, sigma, and shift
coefficients and cutoff distance for this pair style can be mixed. Shift
is always mixed via an *arithmetic* rule. The other coefficients are
mixed according to the pair_modify mix value. The default mix value is
*geometric*. See the \"pair_modify\" command for details.

This pair style supports 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 supports the [pair_modify](pair_modify) tail option for
adding a long-range tail correction to the energy and pressure of the
pair interaction.

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.

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

none

## Related commands

[pair_coeff](pair_coeff)

## Default

none