.. index:: bond_style fene/expand

bond_style fene/expand command
==============================

bond_style fene/expand/omp command
==================================

Syntax
""""""

.. parsed-literal::

   bond_style fene/expand

Examples
""""""""

.. parsed-literal::

   bond_style fene/expand
   bond_coeff 1 30.0 1.5 1.0 1.0 0.5

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

The *fene/expand* bond style uses the potential

.. image:: Eqs/bond_fene_expand.jpg
   :align: center

to define a finite extensible nonlinear elastic (FENE) potential
:ref:`(Kremer) <feneexpand-Kremer>`, used for bead-spring polymer models.  The first
term is attractive, the 2nd Lennard-Jones term is repulsive.

The *fene/expand* bond style is similar to *fene* except that an extra
shift factor of delta (positive or negative) is added to *r* to
effectively change the bead size of the bonded atoms.  The first term
now extends to R0 + delta and the 2nd term is cutoff at 2^(1/6) sigma
+ delta.

The following coefficients must be defined for each bond type via the
:doc:`bond_coeff <bond_coeff>` command as in the example above, or in
the data file or restart files read by the :doc:`read_data <read_data>`
or :doc:`read_restart <read_restart>` commands:

* K (energy/distance^2)
* R0 (distance)
* epsilon (energy)
* sigma (distance)
* delta (distance)


----------


Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in :doc:`Section_accelerate <Section_accelerate>`
of the manual.  The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.

These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
KOKKOS, USER-OMP and OPT packages, respectively.  They are only
enabled if LAMMPS was built with those packages.  See the :ref:`Making LAMMPS <start_3>` section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
use the :doc:`suffix <suffix>` command in your input script.

See :doc:`Section_accelerate <Section_accelerate>` of the manual for
more instructions on how to use the accelerated styles effectively.


----------


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


This bond style can only be used if LAMMPS was built with the
MOLECULE package (which it is by default).  See the :ref:`Making LAMMPS <start_3>` section for more info on packages.

You typically should specify `special_bonds fene <special_bonds.html">`_
or :doc:`special_bonds lj/coul 0 1 1 <special_bonds>` to use this bond
style.  LAMMPS will issue a warning it that's not the case.

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

:doc:`bond_coeff <bond_coeff>`, :doc:`delete_bonds <delete_bonds>`

**Default:** none


----------


.. _feneexpand-Kremer:



**(Kremer)** Kremer, Grest, J Chem Phys, 92, 5057 (1990).


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm