.. index:: improper_coeff

improper_coeff command
======================

Syntax
""""""

.. code-block:: LAMMPS

   improper_coeff N args

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

Examples
""""""""

.. code-block:: LAMMPS

   improper_coeff 1 300.0 0.0
   improper_coeff * 80.2 -1 2
   improper_coeff *4 80.2 -1 2

   labelmap improper 1 benzene
   improper_coeff benzene 300.0 0.0

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

Specify the improper force field coefficients for one or more improper
types.  The number and meaning of the coefficients depends on the
improper style.  Improper 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 a
type label, which is an alphanumeric string defined by the
:doc:`labelmap <labelmap>` command or in a 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 improper types.  This takes the form "\*" or
"\*n" or "n\*" or "m\*n".  If :math:`N` = the number of improper 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 an improper_coeff command can override a previous
setting for the same improper type.  For example, these commands set the
coeffs for all improper types, then overwrite the coeffs for just
improper type 2:

.. code-block:: LAMMPS

   improper_coeff * 300.0 0.0
   improper_coeff 2 50.0 0.0

A line in a data file that specifies improper coefficients uses the
exact same format as the arguments of the improper_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 "Improper Coeffs" section of a data file, the line
that corresponds to the first example above would be listed as

.. parsed-literal::

   1 300.0 0.0

The :doc:`improper_style class2 <improper_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.

----------

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

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

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

:doc:`improper_style <improper_style>`

Default
"""""""

none
