# improper_style sqdistharm command

## Syntax

``` LAMMPS
improper_style sqdistharm
```

## Examples

``` LAMMPS
improper_style sqdistharm
improper_coeff 1 50.0 0.1
```

## Description

The *sqdistharm* improper style uses the potential

$$E = K (d^2 - {d_0}^2)^2$$

where $d$ is the distance between the central atom and the plane formed
by the other three atoms. If the 4 atoms in an improper quadruplet
(listed in the data file read by the [read_data](read_data) command) are
ordered I,J,K,L then the L-atom is assumed to be the central atom. Note
that this is different from the convention used in the improper_style
distance.

The following coefficients must be defined for each improper type via
the improper_coeff command as in the example above, or in the data file
or restart files read by the read_data or read_restart commands:

-   $K$ (energy/distance\^4)
-   ${d_0}^2$ (distance\^2)

Note that ${d_0}^2$ (in units distance\^2) has be provided and not
$d_0$.

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

## Restrictions

This improper style can only be used if LAMMPS was built with the
MOLECULE package. See the [Build package](Build_package) doc page for
more info.

## Related commands

[improper_coeff](improper_coeff)

## Default

none