# improper_style command

## Syntax

``` LAMMPS
improper_style style
```

-   style = *none* or *hybrid* or *class2* or *cvff* or *harmonic*

## Examples

``` LAMMPS
improper_style harmonic
improper_style cvff
improper_style hybrid cvff harmonic
```

## Description

Set the formula(s) LAMMPS uses to compute improper interactions between
quadruplets of atoms, which remain in force for the duration of the
simulation. The list of improper quadruplets is read in by a
[read_data](read_data) or [read_restart](read_restart) command from a
data or restart file. Note that the ordering of the 4 atoms in an
improper quadruplet determines the definition of the improper angle used
in the formula for each style. See the doc pages of individual styles
for details.

Hybrid models where impropers are computed using different improper
potentials can be setup using the *hybrid* improper style.

The coefficients associated with an improper style can be specified in a
data or restart file or via the [improper_coeff](improper_coeff)
command.

All improper potentials store their coefficient data in binary restart
files which means improper_style and [improper_coeff](improper_coeff)
commands do not need to be re-specified in an input script that restarts
a simulation. See the [read_restart](read_restart) command for details
on how to do this. The one exception is that improper_style *hybrid*
only stores the list of sub-styles in the restart file; improper
coefficients need to be re-specified.

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

When both an improper and pair style is defined, the
[special_bonds](special_bonds) command often needs to be used to turn
off (or weight) the pairwise interaction that would otherwise exist
between a group of 4 bonded atoms.
::::

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

Here is an alphabetic list of improper styles defined in LAMMPS. Click
on the style to display the formula it computes and coefficients
specified by the associated [improper_coeff](improper_coeff) command.

Click on the style to display the formula it computes, any additional
arguments specified in the improper_style command, and coefficients
specified by the associated [improper_coeff](improper_coeff) command.

There are also additional accelerated pair styles included in the LAMMPS
distribution for faster performance on CPUs, GPUs, and KNLs. The
individual style names on the [Commands improper](improper) page are
followed by one or more of (g,i,k,o,t) to indicate which accelerated
styles exist.

-   [none](improper_none) - turn off improper interactions
-   [zero](improper_zero) - topology but no interactions
-   [hybrid](improper_hybrid) - define multiple styles of improper
    interactions
-   [amoeba](improper_amoeba) - AMOEBA out-of-plane improper
-   [class2](improper_class2) - COMPASS (class 2) improper
-   [cossq](improper_cossq) - improper with a cosine squared term
-   [cvff](improper_cvff) - CVFF improper
-   [distance](improper_distance) - improper based on distance between
    atom planes
-   [distharm](improper_distharm) - improper that is harmonic in the
    out-of-plane distance
-   [fourier](improper_fourier) - improper with multiple cosine terms
-   [harmonic](improper_harmonic) - harmonic improper
-   [inversion/harmonic](improper_inversion_harmonic) - harmonic
    improper with Wilson-Decius out-of-plane definition
-   [ring](improper_ring) - improper which prevents planar conformations
-   [umbrella](improper_umbrella) - DREIDING improper

[sqdistharm](improper_sqdistharm) - improper that is harmonic in the
square of the out-of-plane distance

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

## Restrictions

Improper styles can only be set for atom_style choices that allow
impropers to be defined.

Most improper styles are part of the MOLECULE package. They are only
enabled if LAMMPS was built with that package. See the [Build
package](Build_package) page for more info. The doc pages for individual
improper potentials tell if it is part of a package.

## Related commands

[improper_coeff](improper_coeff)

## Default

``` LAMMPS
improper_style none
```