# 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.

Note that there are also additional improper styles submitted by users which are included in the LAMMPS distribution. The list of these with links to the individual styles are given in the improper section of this page.

## 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 Making LAMMPS section for more info on packages. The doc pages for individual improper potentials tell if it is part of a package.

## Default

improper_style none