$$\renewcommand{\AA}{\text{Å}}$$

# 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.

## 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).