# bond_style fene command

Accelerator Variants: *fene/intel*, *fene/kk*, *fene/omp*

# bond_style fene/nm command

## Syntax

``` LAMMPS
bond_style fene
bond_style fene/nm
```

## Examples

``` LAMMPS
bond_style fene
bond_coeff 1 30.0 1.5 1.0 1.0

bond_style fene/nm
bond_coeff 1 2.25344 1.5 1.0 1.12246 2 6
```

## Description

The *fene* bond style uses the potential

$$E = -0.5 K R_0^2  \ln \left[ 1 - \left(\frac{r}{R_0}\right)^2\right] + 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right] + \epsilon$$

to define a finite extensible nonlinear elastic (FENE) potential
[(Kremer)](fene-Kremer), used for bead-spring polymer models. The first
term is attractive, the second Lennard-Jones term is repulsive. The
first term extends to $R_0$, the maximum extent of the bond. The second
term is cutoff at $2^\frac{1}{6} \sigma$, the minimum of the LJ
potential.

The *fene/nm* bond style substitutes the standard LJ potential with the
generalized LJ potential in the same form as in pair style
[nm/cut](pair_nm). The bond energy is then given by

$$E = -0.5 K R_0^2  \ln \left[ 1 - \left(\frac{r}{R_0}\right)^2\right] + \frac{E_0}{(n-m)} \left[ m \left(\frac{r_0}{r}\right)^n - n \left(\frac{r_0}{r}\right)^m \right]$$

Similar to the *fene* style, the generalized Lennard-Jones is cut off at
the potential minimum, $r_0$, to be repulsive only. The following
coefficients must be defined for each bond type via the
[bond_coeff](bond_coeff) command as in the example above, or in the data
file or restart files read by the [read_data](read_data) or
[read_restart](read_restart) commands:

-   $K$ (energy/distance\^2)
-   $R_0$ (distance)
-   $\epsilon$ (energy)
-   $\sigma$ (distance)

For the *fene/nm* style, the following coefficients are used. Please
note, that the standard LJ potential and thus the regular FENE potential
is recovered for (n=12 m=6) and $r_0 = 2^\frac{1}{6} \sigma$.

-   $K$ (energy/distance\^2)
-   $R_0$ (distance)
-   $E_0$ (energy)
-   $r_0$ (distance)
-   $n$ (unitless)
-   $m$ (unitless)

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

Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the [Accelerator packages](Speed_packages)
page. The accelerated styles take the same arguments and should produce
the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and
OPT packages, respectively. They are only enabled if LAMMPS was built
with those packages. See the [Build package](Build_package) page for
more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the [-suffix command-line
switch](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_packages) page for more
instructions on how to use the accelerated styles effectively.

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

## Restrictions

The *fene* bond style can only be used if LAMMPS was built with the
MOLECULE package; the *fene/nm* bond style can only be used if LAMMPS
was built with the EXTRA-MOLECULE package. See the [Build
package](Build_package) page for more info.

You typically should specify [special_bonds fene](special_bonds) or
[special_bonds lj/coul 0 1 1](special_bonds) to use this bond style.
LAMMPS will issue a warning it that\'s not the case.

## Related commands

[bond_coeff](bond_coeff), [delete_bonds](delete_bonds), [pair style
lj/cut](pair_lj), [pair style nm/cut](pair_nm).

## Default

none

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

::: {#fene-Kremer}
**(Kremer)** Kremer, Grest, J Chem Phys, 92, 5057 (1990).
:::