\(\renewcommand{\AA}{\text{Å}}\)

# improper_style distance command

## Syntax

    improper_style distance

## Examples

    improper_style distance
    improper_coeff 1 80.0 100.0

## Description

The distance improper style uses the potential

\[E = K_2 d^2 + K_4 d^4\]

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 command) are ordered I,J,K,L then the I-atom is assumed to be the central atom.

Note that defining 4 atoms to interact in this way, does not mean that bonds necessarily exist between I-J, J-K, or K-L, as they would in a linear dihedral. Normally, the bonds I-J, I-K, I-L would exist for an improper to be defined between the 4 atoms.

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:

- \(K_2\) (energy/distance^2)

- \(K_4\) (energy/distance^4)

## Symmetry convention

For the distance improper style, the first atom in the quadruplet is the atom of symmetry; all other atoms are considered interchangeable. This convention is relevant for operations that require knowledge of how atoms are ordered, such as automatic assignment of new improper types by fix bond/react.

## Restrictions

This improper style can only be used if LAMMPS was built with the EXTRA-MOLECULE package. See the Build package doc page for more info.

## Related commands

improper_coeff

## Default

none