.. index:: angle_style dipole

angle_style dipole command
==========================

angle_style dipole/omp command
==============================

Syntax
""""""

.. parsed-literal::

   angle_style dipole

Examples
""""""""

.. parsed-literal::

   angle_style dipole
   angle_coeff 6 2.1 180.0

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

The *dipole* angle style is used to control the orientation of a dipolar
atom within a molecule :ref:`(Orsi) <Orsi>`. Specifically, the *dipole* angle 
style restrains the orientation of a point dipole mu_j (embedded in atom 
'j') with respect to a reference (bond) vector r_ij = r_i - r_j, where 'i' 
is another atom of the same molecule (typically, 'i' and 'j' are also 
covalently bonded).

It is convenient to define an angle gamma between the 'free' vector mu_j
and the reference (bond) vector r_ij:

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

The *dipole* angle style uses the potential:

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

where K is a rigidity constant and gamma0 is an equilibrium (reference)
angle.

The torque on the dipole can be obtained by differentiating the 
potential using the 'chain rule' as in appendix C.3 of 
:ref:`(Allen) <Allen>`:

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

Example: if gamma0 is set to 0 degrees, the torque generated by
the potential will tend to align the dipole along the reference 
direction defined by the (bond) vector r_ij (in other words, mu_j is
restrained to point towards atom 'i').

The dipolar torque T_j must be counterbalanced in order to conserve 
the local angular momentum. This is achieved via an additional force 
couple generating a torque equivalent to the opposite of T_j:

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

where F_i and F_j are applied on atoms i and j, respectively.

The following coefficients must be defined for each angle type via the
:doc:`angle_coeff <angle_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)
* gamma0 (degrees)


----------


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_6>` 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 angle style can only be used if LAMMPS was built with the
USER-MISC package.  See the :ref:`Making LAMMPS <2_3>`
section for more info on packages.

.. note::

   In the "Angles" section of the data file, the atom ID 'j'
   corresponding to the dipole to restrain must come before the atom ID
   of the reference atom 'i'. A third atom ID 'k' must also be provided,
   although 'k' is just a 'dummy' atom which can be any atom; it may be
   useful to choose a convention (e.g., 'k'='i') and adhere to it.  For
   example, if ID=1 for the dipolar atom to restrain, and ID=2 for the
   reference atom, the corresponding line in the "Angles" section of the
   data file would read: X X 1 2 2

The "newton" command for intramolecular interactions must be "on"
(which is the default).

This angle style should not be used with SHAKE.

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

:doc:`angle_coeff <angle_coeff>`, :doc:`angle_hybrid <angle_hybrid>`

**Default:** none


----------


.. _Orsi:



**(Orsi)** Orsi & Essex, The ELBA force field for coarse-grain modeling of 
lipid membranes, PloS ONE 6(12): e28637, 2011.

.. _Allen:



**(Allen)** Allen & Tildesley, Computer Simulation of Liquids,
Clarendon Press, Oxford, 1987.


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