pair_style thole command — LAMMPS 15 May 2015 version documentation

# pair_style thole command

## Syntax

    pair_style thole damp cutoff

- thole = style name
- damp = global damping parameter
- cutoff = global cutoff

## Examples

    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 thermalized Drude oscillator model in LAMMPS and polarizable models in LAMMPS are discussed in this Section.

The thole pair style should be used as a sub-style within in the pair_hybrid/overlay 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

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 (Noskov) of the dipole screening scheme originally proposed by Thole. The scaling coefficient (s_{ij} ) is determined by the polarizability of the atoms, ( alpha_i ), and by a Thole damping parameter ( 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.

begin{equation} s_{ij} = frac{ a_{ij} }{ (alpha_{ij})^{1/3} } = frac{ (a_i + a_j)/2 }{ [(alpha_ialpha_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, ( q_{D,i} ), and opposite charges, ( -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 ( q_i ), but only to the ( -q_{D,i} ) part of it.

The interactions between core charges are subject to the weighting factors set by the 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 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 Making LAMMPS section for more info.

This pair_style should currently not be used with the charmm dihedral style if the latter 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

fix drude, fix langevin/drude, fix drude/transform, compute temp/drude

Default: none

(Noskov) Noskov, Lamoureux and Roux, J Phys Chem B, 109, 6705 (2005).

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