\(\renewcommand{\AA}{\text{Å}}\)

# fix rhok command

## Syntax

    fix ID group-ID rhok nx ny nz K a

- ID, group-ID are documented in fix command

- nx, ny, nz = k-vector of collective density field

- K = spring constant of bias potential

- a = anchor point of bias potential

## Examples

    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

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

as described in (Pedersen).

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

The 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. The default setting for this fix is fix_modify energy no.

This fix computes a global scalar which can be accessed by various output commands. 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 command.

This fix is not invoked during energy minimization.

## 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 page for more info.

## Related commands

thermo_style

## Default

none

(Pedersen) Pedersen, J. Chem. Phys., 139, 104102 (2013).