# pair_style buck6d/coul/gauss/dsf command

# pair_style buck6d/coul/gauss/long command

## Syntax

``` LAMMPS
pair_style style args
```

-   style = *buck6d/coul/gauss/dsf* or *buck6d/coul/gauss/long*
-   args = list of arguments for a particular style

<!-- -->

    *buck6d/coul/gauss/dsf* args = smooth cutoff (cutoff2)
      smooth  = smoothing onset within Buckingham cutoff (ratio)
      cutoff  = global cutoff for Buckingham (and Coulombic if only 1 arg) (distance units)
      cutoff2 = global cutoff for Coulombic (optional) (distance units)
    *buck6d/coul/gauss/long* args = smooth smooth2 cutoff (cutoff2)
      smooth   = smoothing onset within Buckingham cutoff (ratio)
      smooth2  = smoothing onset within Coulombic cutoff (ratio)
      cutoff   = global cutoff for Buckingham (and Coulombic if only 1 arg) (distance units)
      cutoff2  = global cutoff for Coulombic (optional) (distance units)

## Examples

``` LAMMPS
pair_style buck6d/coul/gauss/dsf    0.9000    12.0000
pair_coeff 1  1  1030.  3.061  457.179  4.521  0.608

pair_style buck6d/coul/gauss/long   0.9000  1.0000  12.0000
pair_coeff 1  1  1030.  3.061  457.179  4.521  0.608
```

## Description

The *buck6d/coul/gauss* styles evaluate vdW and Coulomb interactions
following the MOF-FF force field after [(Schmid)](Schmid). The vdW term
of the *buck6d* styles computes a dispersion damped Buckingham
potential:

$$\begin{aligned}
E = A e^{-\kappa r} - \frac{C}{r^6} \cdot \frac{1}{1 + D r^{14}} \qquad r < r_c \\
\end{aligned}$$

where A and C are a force constant, $\kappa$ is an ionic-pair dependent
reciprocal length parameter, D is a dispersion correction parameter, and
the cutoff $r_c$ truncates the interaction distance. The first term in
the potential corresponds to the Buckingham repulsion term and the
second term to the dispersion attraction with a damping correction
analog to the Grimme correction used in DFT. The latter corrects for
artifacts occurring at short distances which become an issue for soft
vdW potentials.

The *buck6d* styles include a smoothing function which is invoked
according to the global smoothing parameter within the specified cutoff.
Hereby a parameter of i.e. 0.9 invokes the smoothing within 90% of the
cutoff. No smoothing is applied at a value of 1.0. For the *gauss/dsf*
style this smoothing is only applicable for the dispersion damped
Buckingham potential. For the *gauss/long* styles the smoothing function
can also be invoked for the real space coulomb interactions which
enforce continuous energies and forces at the cutoff.

Both styles *buck6d/coul/gauss/dsf* and *buck6d/coul/gauss/long*
evaluate a Coulomb potential using spherical Gaussian type charge
distributions which effectively dampen electrostatic interactions for
high charges at close distances. The electrostatic potential is thus
evaluated as:

$$E = \frac{C_{q_i q_j}}{\epsilon r_{ij}}\,\, \textrm{erf}\left(\alpha_{ij} r_{ij}\right)\quad\quad\quad r < r_c$$

where C is an energy-conversion constant, $q_i$ and $q_j$ are the
charges on the 2 atoms, epsilon is the dielectric constant which can be
set by the [dielectric](dielectric) command, $\alpha$ is the ion pair
dependent damping parameter and erf() is the error-function. The cutoff
$r_c$ truncates the interaction distance.

The style *buck6d/coul/gauss/dsf* computes the Coulomb interaction via
the damped shifted force model described in [(Fennell)](Fennell)
approximating an Ewald sum similar to the [pair coul/dsf](pair_coul)
styles. In *buck6d/coul/gauss/long* an additional damping factor is
applied to the Coulombic term so it can be used in conjunction with the
[kspace_style](kspace_style) command and its *ewald* or *pppm* options.
The Coulombic cutoff in this case separates the real and reciprocal
space evaluation of the Ewald sum.

If one cutoff is specified it is used for both the vdW and Coulomb
terms. If two cutoffs are specified, the first is used as the cutoff for
the vdW terms, and the second is the cutoff for the Coulombic term.

The following coefficients must be defined for each pair of atoms types
via the [pair_coeff](pair_coeff) command as in the examples above, or in
the data file or restart files read by the [read_data](read_data) or
[read_restart](read_restart) commands:

-   A (energy units)
-   $\rho$ (distance\^-1 units)
-   C (energy-distance\^6 units)
-   D (distance\^14 units)
-   $\alpha$ (distance\^-1 units)
-   cutoff (distance units)

The second coefficient, $\rho$, must be greater than zero. The latter
coefficient is optional. If not specified, the global vdW cutoff is
used.

------------------------------------------------------------------------

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

These pair styles do not support mixing. Thus, coefficients for all I,J
pairs must be specified explicitly.

These styles do not support the [pair_modify](pair_modify) shift option
for the energy. Instead the smoothing function should be applied by
setting the global smoothing parameter to a value \< 1.0.

These styles write their information to [binary restart files](restart),
so pair_style and pair_coeff commands do not need to be specified in an
input script that reads a restart file.

## Restrictions

These styles are part of the MOFFF package. They are only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

## Related commands

[pair_coeff](pair_coeff)

## Default

none

:::: {#Schmid}
::: {#Fennell}
**(Schmid)** S. Bureekaew, S. Amirjalayer, M. Tafipolsky, C.
Spickermann, T.K. Roy and R. Schmid, Phys. Status Solidi B, 6, 1128
(2013).
:::
::::

**(Fennell)** C. J. Fennell, J. D. Gezelter, J Chem Phys, 124, 234104
(2006).