# improper_style command

## Syntax

improper_style style

• style = none or hybrid or class2 or cvff or harmonic

## Examples

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 or 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 command.

All improper potentials store their coefficient data in binary restart files which means improper_style and improper_coeff commands do not need to be re-specified in an input script that restarts a simulation. See the 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

When both an improper and pair style is defined, the 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 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 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 doc page are followed by one or more of (g,i,k,o,t) to indicate which accelerated styles exist.

• none - turn off improper interactions

• zero - topology but no interactions

• hybrid - define multiple styles of improper interactions

• class2 - COMPASS (class 2) improper

• cossq - improper with a cosine squared term

• cvff - CVFF improper

• distance - improper based on distance between atom planes

• distharm - improper that is harmonic in the out-of-plane distance

• fourier - improper with multiple cosine terms

• harmonic - harmonic improper

• inversion/harmonic - harmonic improper with Wilson-Decius out-of-plane definition

• ring - improper which prevents planar conformations

• umbrella - DREIDING 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 doc page for more info. The doc pages for individual improper potentials tell if it is part of a package.

## Default

improper_style none