\(\renewcommand{\AA}{\text{Å}}\)

# improper_coeff command

## Syntax

    improper_coeff N args

- N = numeric improper type (see asterisk form below), or type label

- args = coefficients for one or more improper types

## Examples

    improper_coeff 1 300.0 0.0
    improper_coeff * 80.2 -1 2
    improper_coeff *4 80.2 -1 2

    labelmap improper 1 benzene
    improper_coeff benzene 300.0 0.0

## Description

Specify the improper force field coefficients for one or more improper types. The number and meaning of the coefficients depends on the improper style. Improper coefficients can also be set in the data file read by the read_data command or in a restart file.

\(N\) can be specified in one of two ways. An explicit numeric value can be used, as in the first example above. Or \(N\) can be a type label, which is an alphanumeric string defined by the labelmap command or in a section of a data file read by the read_data command.

For numeric values only, a wild-card asterisk can be used to set the coefficients for multiple improper types. This takes the form “*” or “*n” or “n*” or “m*n”. If \(N\) = the number of improper types, then an asterisk with no numeric values means all types from 1 to \(N\). A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to \(N\) (inclusive). A middle asterisk means all types from m to n (inclusive).

Note that using an improper_coeff command can override a previous setting for the same improper type. For example, these commands set the coeffs for all improper types, then overwrite the coeffs for just improper type 2:

    improper_coeff * 300.0 0.0
    improper_coeff 2 50.0 0.0

A line in a data file that specifies improper coefficients uses the exact same format as the arguments of the improper_coeff command in an input script, except that wild-card asterisks should not be used since coefficients for all \(N\) types must be listed in the file. For example, under the “Improper Coeffs” section of a data file, the line that corresponds to the first example above would be listed as

    1 300.0 0.0

The improper_style class2 is an exception to this rule, in that an additional argument is used in the input script to allow specification of the cross-term coefficients. See its doc page for details.

The list of all improper styles defined in LAMMPS is given on the improper_style doc page. They are also listed in more compact form on the Commands improper doc page.

On either of those pages, click on the style to display the formula it computes and its coefficients as specified by the associated improper_coeff command.

## Restrictions

This command must come after the simulation box is defined by a read_data, read_restart, or create_box command.

An improper style must be defined before any improper coefficients are set, either in the input script or in a data file.

## Related commands

improper_style

## Default

none