\(\renewcommand{\AA}{\text{Å}}\)

# pair_style coul/tt command

## Syntax

    pair_style style args

- style = coul/tt

- args = list of arguments for a particular style

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

## Examples

    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

Example input scripts available: examples/PACKAGES/drude

## 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 pair_style hybrid/overlay 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

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

This function results from an adaptation to the Coulomb interaction (Salanne) of the damping function originally proposed by Tang Toennies for van der Waals interactions.

The polynomial takes the degree 4 for damping the Coulomb interaction. The parameters \(b_{ij}\) and \(c_{ij}\) could be determined from first-principle calculations for small, mainly mono-atomic, ions (Salanne), 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 \(q_{D,i}\) on a Drude particle or \(-q_{D,i}\) on the respective Drude core)) and a charge on a non-polarizable atom, \(q_{j}\).

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

- \(b_{ij}\)

- \(c_{ij}\)

- 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 DRUDE package. They are only enabled if LAMMPS was built with that package. See the Build package page for more info.

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

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

## Default

none

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

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

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