dihedral_style harmonic dihedral_style multi/harmonic dihedral_style hybrid harmonic charmm
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. 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.
IMPORTANT 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 list of these with links to the individual styles are given in the dihedral section of this page.
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.