# pair_style drip command

## Syntax

``` LAMMPS
pair_style hybrid/overlay drip [styles ...]
```

-   styles = other styles to be overlayed with drip (optional)

## Examples

``` LAMMPS
pair_style hybrid/overlay drip
pair_coeff * * none
pair_coeff * * drip  C.drip  C

pair_style hybrid/overlay drip rebo
pair_coeff * * drip  C.drip     C
pair_coeff * * rebo  CH.airebo  C

pair_style hybrid/overlay drip rebo
pair_coeff * * drip  C.drip     C NULL
pair_coeff * * rebo  CH.airebo  C H
```

## Description

Style *drip* computes the interlayer interactions of layered materials
using the dihedral-angle-corrected registry-dependent (DRIP) potential
as described in [(Wen)](Wen2018), which is based on the
[(Kolmogorov)](Kolmogorov2005) potential and provides an improved
prediction for forces. The total potential energy of a system is

$$\begin{aligned}
E = & \frac{1}{2} \sum_{i} \sum_{j\notin\text{layer}\,i} \phi_{ij} \\
\phi_{ij} = &f_\text{c}(x_r) \left[ e^{-\lambda(r_{ij} - z_0 )} \left[C+f(\rho_{ij})+  g(\rho_{ij}, \{\alpha_{ij}^{(m)}\}) \right]- A\left (\frac{z_0}{r_{ij}} \right)^6 \right]
\end{aligned}$$

where the $r^{-6}$ term models the attractive London dispersion, the
exponential term is designed to capture the registry effect due to
overlapping *pi* bonds, and *fc* is a cutoff function.

This potential (DRIP) only provides the interlayer interactions between
graphene layers. So, to perform a realistic simulation, it should be
used in combination with an intralayer potential such as
[REBO](pair_airebo) and [Tersoff](pair_tersoff). To keep the intralayer
interactions unaffected, we should avoid applying DRIP to contribute
energy to intralayer interactions. This can be achieved by assigning
different molecular IDs to atoms in different layers, and DRIP is
implemented such that only atoms with different molecular ID can
interact with each other. For this purpose, [atom style](atom_style)
\"molecular\" or \"full\" has to be used.

On the other way around, [REBO](pair_airebo) ([Tersoff](pair_tersoff) or
any other potential used to provide the intralayer interactions) should
not interfere with the interlayer interactions described by DRIP. This
is typically automatically achieved using the commands provided in the
*Examples* section above, since the cutoff distance for carbon-carbon
interaction in the intralayer potentials (e.g. 2 Angstrom for
[REBO](pair_airebo)) is much smaller than the equilibrium layer distance
of graphene layers (about 3.4 Angstrom). If you want, you can enforce
this by assigning different atom types to atoms in different layers, and
apply an intralayer potential to one atom type. See
[pair_hybrid](pair_hybrid) for details.

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

The [pair_coeff](pair_coeff) command for DRIP takes *4+N* arguments,
where *N* is the number of LAMMPS atom types. The fist three arguments
must be fixed to be *\* \* drip*, the fourth argument is the path to the
DRIP parameter file, and the remaining N arguments specifying the
mapping between element in the parameter file and atom types. For
example, if your LAMMPS simulation has 3 atom types and you want all of
them to be C, you would use the following pair_coeff command:

``` LAMMPS
pair_coeff * * drip  C.drip  C C C
```

If a mapping value is specified as NULL, the mapping is not performed.
This could be useful when DRIP is used to model part of the system where
other element exists. Suppose you have a hydrocarbon system, with C of
atom type 1 and H of atom type 2, you can use the following command to
inform DRIP not to model H atoms:

``` LAMMPS
pair_style hybrid/overlay drip rebo
pair_coeff * * drip  C.drip     C NULL
pair_coeff * * rebo  CH.airebo  C H
```

:::: note
::: title
Note
:::

The potential parameters developed in [(Wen)](Wen2018) are provided with
LAMMPS (see the \"potentials\" directory). Besides those in
[Wen](Wen2018), an additional parameter \"normal_cutoff\", specific to
the LAMMPS implementation, is used to find the three nearest neighbors
of an atom to construct the normal.
::::

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

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

This pair style does not support the pair_modify mix, shift, table, and
tail options.

This pair style does not write their information to binary restart
files, since it is stored in potential files. Thus, you need to
re-specify the pair_style and pair_coeff commands in an input script
that reads a restart file.

## Restrictions

This pair style is part of the INTERLAYER package. It is only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

This pair style requires the [newton](newton) setting to be \"on\" for
pair interactions.

The *C.drip* parameter file provided with LAMMPS (see the \"potentials\"
directory) is parameterized for metal [units](units). You can use the
DRIP potential with any LAMMPS units, but you would need to create your
own custom parameter file with coefficients listed in the appropriate
units, if your simulation does not use \"metal\" units.

## Related commands

[pair_style lebedeva_z](pair_lebedeva_z), [pair_style
kolmogorov/crespi/z](pair_kolmogorov_crespi_z), [pair_style
kolmogorov/crespi/full](pair_kolmogorov_crespi_full), [pair_style
ilp/graphene/hbn](pair_ilp_graphene_hbn).

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

::: {#Wen2018}
**(Wen)** M. Wen, S. Carr, S. Fang, E. Kaxiras, and E. B. Tadmor, Phys.
Rev. B, 98, 235404 (2018)
:::

::: {#Kolmogorov2005}
**(Kolmogorov)** A. N. Kolmogorov, V. H. Crespi, Phys. Rev. B 71, 235415
(2005)
:::