.. index:: pair_style coul/tt

pair_style coul/tt command
==========================

Syntax
""""""

.. code-block:: LAMMPS

   pair_style style args

* style = *coul/tt*
* args = list of arguments for a particular style

.. parsed-literal::

     *coul/tt* args = n cutoff
       n = degree of polynomial
       cutoff = global cutoff (distance units)

Examples
""""""""

.. code-block:: LAMMPS

   pair_style hybrid/overlay ... coul/tt 4 12.0
   pair_coeff 1 2  coul/tt 4.5 1.0
   pair_coeff 1 2  coul/tt 4.0 1.0 4 12.0
   pair_coeff 1 3* coul/tt 4.5 1.0 4


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

The *coul/tt* pair style is meant to be used with force fields that
include explicit polarization through Drude dipoles.

The *coul/tt* pair style should be used as a sub-style within in the
:doc:`pair_style hybrid/overlay <pair_hybrid>` command, in conjunction with a
main pair style including Coulomb interactions and *thole* pair style,
or with *lj/cut/thole/long* pair style that is equivalent to the combination
of preceding two.

The *coul/tt* pair styles compute the charge-dipole Coulomb interaction damped
at short distances by a function

.. math::

  f_{n,ij}(r) = 1 - c_{ij} \cdot e^{-b_{ij} r} \sum_{k=0}^n \frac{(b_{ij} r)^k}{k!}

This function results from an adaptation to the Coulomb interaction :ref:`(Salanne)
<Salanne1>` of the damping function originally proposed
by :ref:`Tang Toennies <TangToennies1>` for van der Waals interactions.

The polynomial takes the degree 4 for damping the Coulomb interaction.
The parameters :math:`b_{ij}` and :math:`c_{ij}` could be determined from
first-principle calculations for small, mainly mono-atomic, ions :ref:`(Salanne)
<Salanne1>`, or else treated as empirical for large molecules.

In pair styles with Drude induced dipoles, this damping function is typically
applied to the interactions between a Drude charge (either :math:`q_{D,i}` on
a Drude particle or :math:`-q_{D,i}` on the respective
Drude core)) and a charge on a non-polarizable atom, :math:`q_{j}`.

The Tang-Toennies function could also be used to damp electrostatic
interactions between the (non-polarizable part of the) charge of a core,
:math:`q_{i}-q_{D,i}`, and the Drude charge of another, :math:`-q_{D,j}`.
The :math:`b_{ij}` and :math:`c_{ij}` are equal  to :math:`b_{ji}` and
:math:`c_{ji}` in the case of core-core interactions.

For pair_style *coul/tt*\ , the following coefficients must be defined for
each pair of atoms types via the :doc:`pair_coeff <pair_coeff>` command
as in the example above.

* :math:`b_{ij}`
* :math:`c_{ij}`
* degree of polynomial (positive integer)
* cutoff (distance units)

The last two coefficients are optional.  If not specified the global
degree of the polynomial or the global cutoff specified in the pair_style
command are used. In order to specify a cutoff (forth argument), the degree of
the polynomial (third argument) must also be specified.

----------

Mixing, shift, table, tail correction, restart, rRESPA info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

The *coul/tt* pair style does not support mixing.  Thus, coefficients
for all I,J pairs must be specified explicitly.

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

These pair styles are part of the USER-DRUDE package. They are only
enabled if LAMMPS was built with that package. See the :doc:`Build package
<Build_package>` doc page for more info.

This pair_style should currently not be used with the :doc:`charmm dihedral
style <dihedral_charmm>` if the latter has non-zero 1-4 weighting
factors. This is because the *coul/tt* pair style does not know which
pairs are 1-4 partners of which dihedrals.

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

:doc:`fix drude <fix_drude>`, :doc:`fix langevin/drude <fix_langevin_drude>`,
:doc:`fix drude/transform <fix_drude_transform>`,
:doc:`compute temp/drude <compute_temp_drude>`,
:doc:`pair_style thole <pair_thole>`

Default
"""""""

none

----------

.. _Thole3:

**(Thole)** Chem Phys, 59, 341 (1981).

.. _Salanne1:

**(Salanne)** Salanne, Rotenberg, Jahn, Vuilleumier, Simon, Christian and Madden, Theor Chem Acc, 131, 1143 (2012).

.. _TangToennies1:

**(Tang and Toennies)** J Chem Phys, 80, 3726 (1984).
