# dihedral_style command

## Syntax

``` LAMMPS
dihedral_style style
```

-   style = *none* or *zero* or *hybrid* or *charmm* or *charmmfsw* or
    *class2* or *cosine/shift/exp* or *fourier* or *harmonic* or *helix*
    or *lepton* or *multi/harmonic* or *nharmonic* or *opls* or
    *spherical* or *table* or *table/cut*

## Examples

``` LAMMPS
dihedral_style harmonic
dihedral_style multi/harmonic
dihedral_style hybrid harmonic charmm
```

## Description

Set the formula(s) LAMMPS uses to compute dihedral interactions between
quadruplets of atoms, which remain in force for the duration of the
simulation. The list of dihedral quadruplets is read in by a
[read_data](read_data) or [read_restart](read_restart) command from a
data or restart file.

Hybrid models where dihedrals are computed using different dihedral
potentials can be setup using the *hybrid* dihedral style.

The coefficients associated with a dihedral style can be specified in a
data or restart file or via the [dihedral_coeff](dihedral_coeff)
command.

All dihedral potentials store their coefficient data in binary restart
files which means dihedral_style and [dihedral_coeff](dihedral_coeff)
commands do not need to be re-specified in an input script that restarts
a simulation. See the [read_restart](read_restart) command for details
on how to do this. The one exception is that dihedral_style *hybrid*
only stores the list of sub-styles in the restart file; dihedral
coefficients need to be re-specified.

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

When both a dihedral and pair style is defined, the
[special_bonds](special_bonds) command often needs to be used to turn
off (or weight) the pairwise interaction that would otherwise exist
between four bonded atoms.
::::

In the formulas listed for each dihedral style, *phi* is the torsional
angle defined by the quadruplet of atoms. This angle has a sign
convention as shown in this diagram:

![image](JPG/dihedral_sign.jpg){.align-center}

where the $I,J,K,L$ ordering of the four atoms that define the dihedral
is from left to right.

This sign convention effects several of the dihedral styles listed below
(e.g., charmm, helix) in the sense that the energy formula depends on
the sign of phi, which may be reflected in the value of the coefficients
you specify.

:::: 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 via the
[dihedral_coeff](dihedral_coeff) command to account for this difference
if necessary.
::::

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

Here is an alphabetic list of dihedral styles defined in LAMMPS. Click
on the style to display the formula it computes and coefficients
specified by the associated [dihedral_coeff](dihedral_coeff) command.

Click on the style to display the formula it computes, any additional
arguments specified in the dihedral_style command, and coefficients
specified by the associated [dihedral_coeff](dihedral_coeff) command.

There are also additional accelerated pair styles included in the LAMMPS
distribution for faster performance on CPUs, GPUs, and KNLs. The
individual style names on the [Commands dihedral](dihedral) page are
followed by one or more of (g,i,k,o,t) to indicate which accelerated
styles exist.

-   [none](dihedral_none) - turn off dihedral interactions
-   [zero](dihedral_zero) - topology but no interactions
-   [hybrid](dihedral_hybrid) - define multiple styles of dihedral
    interactions
-   [charmm](dihedral_charmm) - CHARMM dihedral
-   [charmmfsw](dihedral_charmm) - CHARMM dihedral with force switching
-   [class2](dihedral_class2) - COMPASS (class 2) dihedral
-   [cosine/shift/exp](dihedral_cosine_shift_exp) - dihedral with
    exponential in spring constant
-   [fourier](dihedral_fourier) - dihedral with multiple cosine terms
-   [harmonic](dihedral_harmonic) - harmonic dihedral
-   [helix](dihedral_helix) - helix dihedral
-   [lepton](dihedral_lepton) - dihedral potential from evaluating a
    string
-   [multi/harmonic](dihedral_multi_harmonic) - dihedral with 5 harmonic
    terms
-   [nharmonic](dihedral_nharmonic) - same as multi-harmonic with N
    terms
-   [opls](dihedral_opls) - OPLS dihedral
-   [quadratic](dihedral_quadratic) - dihedral with quadratic term in
    angle
-   [spherical](dihedral_spherical) - dihedral which includes angle
    terms to avoid singularities
-   [table](dihedral_table) - tabulated dihedral
-   [table/cut](dihedral_table) - tabulated dihedral with analytic
    cutoff

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

## Restrictions

Dihedral styles can only be set for atom styles that allow dihedrals to
be defined.

Most dihedral styles are part of the MOLECULE package. They are only
enabled if LAMMPS was built with that package. See the [Build
package](Build_package) page for more info. The doc pages for individual
dihedral potentials tell if it is part of a package.

## Related commands

[dihedral_coeff](dihedral_coeff)

## Default

dihedral_style none