# dihedral_coeff command

## Syntax

``` LAMMPS
dihedral_coeff N args
```

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

## Examples

``` LAMMPS
dihedral_coeff 1 80.0 1 3
dihedral_coeff * 80.0 1 3 0.5
dihedral_coeff 2* 80.0 1 3 0.5

labelmap dihedral 1 backbone
dihedral_coeff backbone 80.0 1 3
```

## Description

Specify the dihedral force field coefficients for one or more dihedral
types. The number and meaning of the coefficients depends on the
dihedral style. Dihedral 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 an alphanumeric
type label, which is a string defined by the [labelmap](labelmap)
command or in a corresponding 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 dihedral types. This takes the form \"\*\" or
\"\*n\" or \"n\*\" or \"m\*n\". If $N$ is the number of dihedral 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 a dihedral_coeff command can override a previous setting
for the same dihedral type. For example, these commands set the coeffs
for all dihedral types, then overwrite the coeffs for just dihedral type
2:

``` LAMMPS
dihedral_coeff * 80.0 1 3
dihedral_coeff 2 200.0 1 3
```

A line in a data file that specifies dihedral coefficients uses the
exact same format as the arguments of the dihedral_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 \"Dihedral Coeffs\" section of a data file, the line that
corresponds to the first example above would be listed as

    1 80.0 1 3

The [dihedral_style class2](dihedral_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.

:::: note
::: title
Note
:::

When comparing the formulas and coefficients for various LAMMPS dihedral
styles with dihedral equations defined by other force fields, note that
some force field implementations divide/multiply the energy prefactor
*K* by the multiple number of torsions that contain the *J*\--\*K\* bond
in an *I*-\*J\*-\*K\*-\*L\* torsion. LAMMPS does not do this (i.e., the
listed dihedral equation applies to each individual dihedral). Thus, you
need to define *K* appropriately to account for this difference, if
necessary.
::::

------------------------------------------------------------------------

The list of all dihedral styles defined in LAMMPS is given on the
[dihedral_style](dihedral_style) doc page. They are also listed in more
compact form on the [Commands dihedral](dihedral) 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
dihedral_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.

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

## Related commands

[dihedral_style](dihedral_style)

## Default

none