# pair_style nb3b/harmonic command

## Syntax

``` LAMMPS
pair_style nb3b/harmonic
```

## Examples

``` LAMMPS
pair_style nb3b/harmonic
pair_coeff * * MgOH.nb3bharmonic Mg O H
```

## Description

This pair style computes a non-bonded 3-body harmonic potential for the
energy E of a system of atoms as

$$E = K (\theta - \theta_0)^2$$

where $\theta_0$ is the equilibrium value of the angle and *K* is a
prefactor. Note that the usual 1/2 factor is included in *K*. The form
of the potential is identical to that used in angle_style *harmonic*,
but in this case, the atoms do not need to be explicitly bonded.

Only a single pair_coeff command is used with this style which specifies
a potential file with parameters for specified 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 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 a file SiC.nb3b.harmonic has potential values for
Si and C. If your LAMMPS simulation has 4 atoms types and you want the
first 3 to be Si, and the fourth to be C, you would use the following
pair_coeff command:

``` LAMMPS
pair_coeff * * SiC.nb3b.harmonic Si Si Si C
```

The first 2 arguments must be \* \* so as to span all LAMMPS atom types.
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
element in the potential file. The final C argument maps LAMMPS atom
type 4 to the C element in the potential file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used when
the 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. An example of a pair_coeff command for use with the *hybrid*
pair style is:

``` LAMMPS
pair_coeff * * nb3b/harmonic MgOH.nb3b.harmonic Mg O H
```

Three-body non-bonded harmonic files in the *potentials* directory of
the LAMMPS distribution have a \".nb3b.harmonic\" suffix. Lines that are
not blank or comments (starting with #) define parameters for a triplet
of elements.

Each entry has six arguments. The first three are atom types as
referenced in the LAMMPS input file. The first argument specifies the
central atom. The fourth argument indicates the *K* parameter. The fifth
argument indicates $\theta_0$. The sixth argument indicates a separation
cutoff in Angstroms.

For a given entry, if the second and third arguments are identical, then
the entry is for a cutoff for the distance between types 1 and 2 (values
for *K* and $\theta_0$ are irrelevant in this case).

For a given entry, if the first three arguments are all different, then
the entry is for the *K* and $\theta_0$ parameters (the cutoff in this
case is irrelevant).

It is required that the potential file contains entries for *all*
permutations of the elements listed in the pair_coeff command. If
certain combinations are not parameterized the corresponding parameters
should be set to zero. The potential file can also contain entries for
additional elements which are not used in a particular simulation;
LAMMPS ignores those entries.

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

## Restrictions

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

## Related commands

[pair_coeff](pair_coeff)

## Default

none