.. index:: improper_style distharm

improper_style distharm command
===============================

Syntax
""""""

.. code-block:: LAMMPS

   improper_style distharm

Examples
""""""""

.. code-block:: LAMMPS

   improper_style distharm
   improper_coeff 1 25.0 0.5

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

The *distharm* improper style uses the potential

.. math::

   E = K (d - d_0)^2

where :math:`d` is the oriented distance between the central atom and the plane formed
by the other three atoms.  If the 4 atoms in an improper quadruplet
(listed in the data file read by the :doc:`read_data <read_data>`
command) are ordered I,J,K,L then the L-atom is assumed to be the
central atom. Note that this is different from the convention used
in the improper_style distance. The distance :math:`d` is oriented and can take
on negative values. This may lead to unwanted behavior if :math:`d_0` is not equal to zero.

The following coefficients must be defined for each improper type via
the improper_coeff command as in the example above, or in the data
file or restart files read by the read_data or read_restart commands:

* :math:`K` (energy/distance\^2)
* :math:`d_0` (distance)

----------

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

This improper style can only be used if LAMMPS was built with the
USER-YAFF package.  See the :doc:`Build package <Build_package>` doc
page for more info.

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

:doc:`improper_coeff <improper_coeff>`

**Default:** none
