# improper_coeff command

## Syntax

``` LAMMPS
improper_coeff N args
```

-   N = numeric improper type (see asterisk form below), or type label
-   args = coefficients for one or more improper types

## Examples

``` LAMMPS
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](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](labelmap)
command or in a section of a data file read by the
[read_data](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:

``` LAMMPS
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](improper_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](improper_style) doc page. They are also listed in more
compact form on the [Commands improper](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_data), [read_restart](read_restart), or
[create_box](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](improper_style)

## Default

none