.. index:: dihedral_coeff

dihedral_coeff command
======================

Syntax
""""""

.. code-block:: LAMMPS

   dihedral_coeff N args

* N = numeric dihedral type (see asterisk form below) or alphanumeric type label
* args = coefficients for one or more dihedral types

Examples
""""""""

.. code-block:: LAMMPS

   dihedral_coeff 1 80.0 1 3
   dihedral_coeff * 80.0 1 3 0.5
   dihedral_coeff 2* 80.0 1 3 0.5

   labelmap dihedral 1 backbone
   dihedral_coeff backbone 80.0 1 3

Description
"""""""""""

Specify the dihedral force field coefficients for one or more dihedral
types.  The number and meaning of the coefficients depends on the
dihedral style.  Dihedral coefficients can also be set in the data file
read by the :doc:`read_data <read_data>` command or in a restart file.

:math:`N` can be specified in one of two ways.  An explicit numeric
value can be used, as in the first example above.  Or :math:`N` can be
an alphanumeric type label, which is a string defined by the
:doc:`labelmap <labelmap>` command or in a corresponding section of a
data file read by the :doc:`read_data <read_data>` command.

For numeric values only, a wild-card asterisk can be used to set the
coefficients for multiple dihedral types.  This takes the form "\*" or
"\*n" or "n\*" or "m\*n".  If :math:`N` is the number of dihedral types,
then an asterisk with no numeric values means all types from 1 to
:math:`N`.  A leading asterisk means all types from 1 to n (inclusive).
A trailing asterisk means all types from n to :math:`N` (inclusive).  A
middle asterisk means all types from m to n (inclusive).

Note that using a dihedral_coeff command can override a previous setting
for the same dihedral type.  For example, these commands set the coeffs
for all dihedral types, then overwrite the coeffs for just dihedral type
2:

.. code-block:: LAMMPS

   dihedral_coeff * 80.0 1 3
   dihedral_coeff 2 200.0 1 3

A line in a data file that specifies dihedral coefficients uses the exact
same format as the arguments of the dihedral_coeff command in an input
script, except that wild-card asterisks should not be used since
coefficients for all :math:`N` types must be listed in the file.  For example,
under the "Dihedral Coeffs" section of a data file, the line that
corresponds to the first example above would be listed as

.. parsed-literal::

   1 80.0 1 3

The :doc:`dihedral_style class2 <dihedral_class2>` is an exception to
this rule, in that an additional argument is used in the input script
to allow specification of the cross-term coefficients.  See its doc
page for details.

.. 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 to account for this
   difference, if necessary.

----------

The list of all dihedral styles defined in LAMMPS is given on the
:doc:`dihedral_style <dihedral_style>` doc page.  They are also listed
in more compact form on the :ref:`Commands dihedral <dihedral>` doc page.

On either of those pages, click on the style to display the formula it
computes and its coefficients as specified by the associated
dihedral_coeff command.

----------

Restrictions
""""""""""""

This command must come after the simulation box is defined by a
:doc:`read_data <read_data>`, :doc:`read_restart <read_restart>`, or
:doc:`create_box <create_box>` command.

A dihedral style must be defined before any dihedral coefficients are
set, either in the input script or in a data file.

Related commands
""""""""""""""""

:doc:`dihedral_style <dihedral_style>`

Default
"""""""

none
