# fix rhok command

## Syntax

    fix ID group-ID rhok nx ny nz K a

-   ID, group-ID are documented in [fix](fix) command
-   nx, ny, nz = k-vector of collective density field
-   K = spring constant of bias potential
-   a = anchor point of bias potential

## Examples

``` LAMMPS
fix bias all rhok 16 0 0 4.0 16.0
fix 1 all npt temp 0.8 0.8 4.0 z 2.2 2.2 8.0
# output of 4 values from fix rhok: U_bias rho_k_RE  rho_k_IM  \|rho_k\|
thermo_style custom step temp pzz lz f_bias f_bias[1] f_bias[2] f_bias[3]
```

## Description

The fix applies a force to atoms given by the potential

$$\begin{aligned}
U  = &  \frac{1}{2} K (|\rho_{\vec{k}}| - a)^2 \\
\rho_{\vec{k}}  = & \sum_j^N \exp(-i\vec{k} \cdot \vec{r}_j )/\sqrt{N} \\
\vec{k}  = & (2\pi n_x /L_x , 2\pi n_y  /L_y , 2\pi n_z/L_z )
\end{aligned}$$

as described in [(Pedersen)](Pedersen).

This field, which biases configurations with long-range order, can be
used to study crystal-liquid interfaces and determine melting
temperatures [(Pedersen)](Pedersen).

An example of using the interface pinning method is located in the
*examples/PACKAGES/rhok* directory.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart).

The [fix_modify](fix_modify) *energy* option is supported by this fix to
add the potential energy calculated by the fix to the global potential
energy of the system as part of [thermodynamic output](thermo_style).
The default setting for this fix is [fix_modify energy no](fix_modify).

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). The scalar is the potential energy
discussed in the preceding paragraph. The scalar stored by this fix is
\"extensive\".

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command.

This fix is not invoked during [energy minimization](minimize).

## Restrictions

This fix is part of the EXTRA-FIX package. It is only enabled if LAMMPS
was built with that package. See the [Build package](Build_package) page
for more info.

## Related commands

[thermo_style](thermo_style)

## Default

none

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

::: {#Pedersen}
**(Pedersen)** Pedersen, J. Chem. Phys., 139, 104102 (2013).
:::