\(\renewcommand{\AA}{\text{Å}}\)

# improper_style sqdistharm command

## Syntax

    improper_style sqdistharm

## Examples

    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 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 \(d_0\).

## Symmetry convention

For the sqdistharm improper style, the fourth 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 MOLECULE package. See the Build package doc page for more info.

## Related commands

improper_coeff

## Default

none