\(\renewcommand{\AA}{\text{Å}}\)

# dihedral_style command¶

## Syntax¶

```
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¶

```
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 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:

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

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.

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 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 page are followed by one or more of (g,i,k,o,t) to indicate which accelerated styles exist.

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

lepton - dihedral potential from evaluating a string

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 page for more info. The doc pages for individual dihedral potentials tell if it is part of a package.

## Default¶

dihedral_style none