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### 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.

IMPORTANT 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.

Here are some important points to take note of when defining the LAMMPS dihedral coefficients in the formulas that follow so that they are compatible with other force fields:

- The LAMMPS convention is that the trans position = 180 degrees, while in some force fields trans = 0 degrees.
- Some force fields reverse the sign convention on d.
- Some force fields divide/multiply K by the number of multiple torsions that contain the j-k bond in an i-j-k-l torsion.
- Some force fields let n be positive or negative which corresponds to d = 1 or -1 for the harmonic style.

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:

- dihedral_style none - turn off dihedral interactions
- dihedral_style hybrid - define multiple styles of dihedral interactions

- dihedral_style charmm - CHARMM dihedral
- dihedral_style class2 - COMPASS (class 2) dihedral
- dihedral_style harmonic - harmonic dihedral
- dihedral_style helix - helix dihedral
- dihedral_style multi/harmonic - multi-harmonic dihedral
- dihedral_style opls - OPLS dihedral

There are also additional dihedral styles submitted by users which are included in the LAMMPS distribution. The list of these with links to the individual styles are given in the dihedral section of this page.

Restrictions:

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

Most dihedral styles are part of the "molecular" package. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info on packages. The doc pages for individual dihedral potentials tell if it is part of a package.

Related commands:

dihedral_coeff

Default:

dihedral_style none