fix rhok command


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


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]


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/USER/misc/rhok directory.


This fix is part of the USER-MISC package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.



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