# pair_style meam/sw/spline command

## Syntax

``` LAMMPS
pair_style meam/sw/spline
```

## Examples

``` LAMMPS
pair_style meam/sw/spline
pair_coeff * * Ti.meam.sw.spline Ti
pair_coeff * * Ti.meam.sw.spline Ti Ti Ti
```

## Description

The *meam/sw/spline* style computes pairwise interactions for metals
using a variant of modified embedded-atom method (MEAM) potentials
[(Lenosky)](Lenosky2) with an additional Stillinger-Weber (SW) term
[(Stillinger)](Stillinger1) in the energy. This form of the potential
was first proposed by Nicklas, Fellinger, and Park [(Nicklas)](Nicklas).
We refer to it as MEAM+SW. The total energy E is given by

$$\begin{aligned}
E & = E_{MEAM} + E_{SW} \\
E_{MEAM} & =  \sum _{IJ} \phi (r_{IJ}) + \sum _{I} U(\rho _I) \\
E_{SW} & =  \sum _{I} \sum _{JK} F(r_{IJ}) \, F(r_{IK}) \, G(\cos(\theta _{JIK})) \\
\rho _I & = \sum _J \rho(r_{IJ}) + \sum _{JK} f(r_{IJ}) \, f(r_{IK}) \, g(\cos(\theta _{JIK}))
\end{aligned}$$

where $\rho_I$ is the density at atom I, $\theta_{JIK}$ is the angle
between atoms J, I, and K centered on atom I. The seven functions
$\phi, F, G, U, \rho, f,$ and *g* are represented by cubic splines.

The cutoffs and the coefficients for these spline functions are listed
in a parameter file which is specified by the [pair_coeff](pair_coeff)
command. Parameter files for different elements are included in the
\"potentials\" directory of the LAMMPS distribution and have a
\".meam.sw.spline\" file suffix. All of these files are parameterized in
terms of LAMMPS [metal units](units).

Note that unlike for other potentials, cutoffs for spline-based MEAM+SW
potentials are not set in the pair_style or pair_coeff command; they are
specified in the potential files themselves.

Unlike the EAM pair style, which retrieves the atomic mass from the
potential file, the spline-based MEAM+SW potentials do not include mass
information; thus you need to use the [mass](mass) command to specify
it.

Only a single pair_coeff command is used with the meam/sw/spline style
which specifies a potential file with parameters for all needed
elements. These are mapped to LAMMPS atom types by specifying N
additional arguments after the filename in the pair_coeff command, where
N is the number of LAMMPS atom types:

-   filename
-   N element names = mapping of spline-based MEAM+SW elements to atom
    types

See the [pair_coeff](pair_coeff) page for alternate ways to specify the
path for the potential file.

As an example, imagine the Ti.meam.sw.spline file has values for Ti. If
your LAMMPS simulation has 3 atoms types and they are all to be treated
with this potential, you would use the following pair_coeff command:

``` LAMMPS
pair_coeff * * Ti.meam.sw.spline Ti Ti Ti
```

The first 2 arguments must be \* \* so as to span all LAMMPS atom types.
The three Ti arguments map LAMMPS atom types 1,2,3 to the Ti element in
the potential file. If a mapping value is specified as NULL, the mapping
is not performed. This can be used when a *meam/sw/spline* potential is
used as part of the hybrid pair style. The NULL values are placeholders
for atom types that will be used with other potentials.

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

The *meam/sw/spline* style currently supports only single-element
MEAM+SW potentials. It may be extended for alloy systems in the future.
::::

Example input scripts that use this pair style are provided in the
examples/PACKAGES/meam_sw_spline directory.

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

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

The pair style does not support multiple element types or mixing. It has
been designed for pure elements only.

This pair style does not support the [pair_modify](pair_modify) shift,
table, and tail options.

The *meam/sw/spline* pair style does not write its information to
[binary restart files](restart), since it is stored in an external
potential parameter file. Thus, you need to re-specify the pair_style
and pair_coeff commands in an input script that reads a restart file.

The *meam/sw/spline* pair style can only be used via the *pair* keyword
of the [run_style respa](run_style) command. They do not support the
*inner*, *middle*, *outer* keywords.

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

## Restrictions

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

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

## Related commands

[pair_coeff](pair_coeff), [pair_style meam](pair_meam), [pair_style
meam/spline](pair_meam_spline)

## Default

none

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

::: {#Lenosky2}
**(Lenosky)** Lenosky, Sadigh, Alonso, Bulatov, de la Rubia, Kim, Voter,
Kress, Modell. Simul. Mater. Sci. Eng. 8, 825 (2000).
:::

::: {#Stillinger1}
**(Stillinger)** Stillinger, Weber, Phys. Rev. B 31, 5262 (1985).
:::

::: {#Nicklas}
**(Nicklas)** The spline-based MEAM+SW format was first devised and used
to develop potentials for bcc transition metals by Jeremy Nicklas,
Michael Fellinger, and Hyoungki Park at The Ohio State University.
:::