# dihedral_style command

## Syntax

```
dihedral_style style
```

- style =
*none*or*hybrid*or*charmm*or*class2*or*harmonic*or*helix*or*multi/harmonic*or*opls*

## Examples

```
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 or 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 command.

All dihedral potentials store their coefficient data in binary restart
files which means dihedral_style and
dihedral_coeff commands do not need to be
re-specified in an input script that restarts a simulation. See the
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

When both a dihedral and pair style is defined, the special_bonds command often needs to be used to turn off (or weight) the pairwise interaction that would otherwise exist between 4 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:

where the I,J,K,L ordering of the 4 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

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 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 command.

Note that there are also additional dihedral styles submitted by users which are included in the LAMMPS distribution. The full list of all dihedral styles is on the Commands bond doc page.

- none - turn off dihedral interactions
- zero - topology but no interactions
- hybrid - define multiple styles of dihedral interactions
- charmm - CHARMM dihedral
- charmmfsw - CHARMM dihedral with force switching
- class2 - COMPASS (class 2) dihedral
- cosine/shift/exp - dihedral with exponential in spring constant
- fourier - dihedral with multiple cosine terms
- harmonic - harmonic dihedral
- helix - helix dihedral
- multi/harmonic - dihedral with 5 harmonic terms
- nharmonic - same as multi-harmonic with N terms
- opls - OPLS dihedral
- quadratic - dihedral with quadratic term in angle
- spherical - dihedral which includes angle terms to avoid singularities
- table - tabulated dihedral
- table/cut - 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 doc page for more info. The doc pages for individual dihedral potentials tell if it is part of a package.

## Default

dihedral_style none