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.. index:: pair_style thole
pair_style thole command
========================
Syntax
""""""
.. parsed-literal::
pair_style thole damp cutoff
* thole = style name
* damp = global damping parameter
* cutoff = global cutoff
Examples
""""""""
.. parsed-literal::
pair_style hybrid/overlay ... thole 2.6 12.0
pair_coeff 1 1 thole 1.0
pair_coeff 1 2 thole 1.0 2.6 10.0
pair_coeff * 2 thole 1.0 2.6
Description
"""""""""""
The *thole* pair style is meant to be used with force fields that
include explicit polarization through Drude dipoles. This link
describes how to use the :doc:`thermalized Drude oscillator model <tutorial_drude>` in LAMMPS and polarizable models in LAMMPS
are discussed in :ref:`this Section <howto_25>`.
The *thole* pair style should be used as a sub-style within in the
:doc:`pair_hybrid/overlay <pair_hybrid>` command, in conjunction with a
main pair style including Coulomb interactions, i.e. any pair style
containing *coul/cut* or *coul/long* in its style name.
The *thole* pair style computes the Coulomb interaction damped at
short distances by a function
.. math::
\begin{equation} T_{ij}(r_{ij}) = 1 - \left( 1 +
\frac{s_{ij} r_{ij} }{2} \right)
\exp \left( - s_{ij} r_{ij} \right) \end{equation}
This function results from an adaptation to point charges
:ref:`(Noskov) <Noskov>` of the dipole screening scheme originally proposed
by :ref:`Thole <Thole>`. The scaling coefficient :math:`s_{ij}` is determined
by the polarizability of the atoms, :math:`\alpha_i`, and by a Thole
damping parameter :math:`a`. This Thole damping parameter usually takes
a value of 2.6, but in certain force fields the value can depend upon
the atom types. The mixing rule for Thole damping parameters is the
arithmetic average, and for polarizabilities the geometric average
between the atom-specific values.
.. math::
\begin{equation} s_{ij} = \frac{ a_{ij} }{
(\alpha_{ij})^{1/3} } = \frac{ (a_i + a_j)/2 }{
[(\alpha_i\alpha_j)^{1/2}]^{1/3} } \end{equation}
The damping function is only applied to the interactions between the
point charges representing the induced dipoles on polarizable sites,
that is, charges on Drude particles, :math:`q_{D,i}`, and opposite
charges, :math:`-q_{D,i}`, located on the respective core particles
(to which each Drude particle is bonded). Therefore, Thole screening
is not applied to the full charge of the core particle :math:`q_i`, but
only to the :math:`-q_{D,i}` part of it.
The interactions between core charges are subject to the weighting
factors set by the :doc:`special_bonds <special_bonds>` command. The
interactions between Drude particles and core charges or
non-polarizable atoms are also subject to these weighting factors. The
Drude particles inherit the 1-2, 1-3 and 1-4 neighbor relations from
their respective cores.
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.
* alpha (distance units^3)
* damp
* cutoff (distance units)
The last two coefficients are optional. If not specified the global
Thole damping parameter or global cutoff specified in the pair_style
command are used. In order to specify a cutoff (third argument) a damp
parameter (second argument) must also be specified.
**Mixing**:
The *thole* 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 :ref:`Making LAMMPS <start_3>` section 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 *thole* 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>`
**Default:** none
----------
.. _Noskov:
**(Noskov)** Noskov, Lamoureux and Roux, J Phys Chem B, 109, 6705 (2005).
.. _Thole:
**(Thole)** Chem Phys, 59, 341 (1981).
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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