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# fix langevin/eff command

## Syntax

    fix ID group-ID langevin/eff Tstart Tstop damp seed keyword values ...

- ID, group-ID are documented in fix command

- langevin/eff = style name of this fix command

- Tstart,Tstop = desired temperature at start/end of run (temperature units)

- damp = damping parameter (time units)

- seed = random number seed to use for white noise (positive integer)

- zero or more keyword/value pairs may be appended

      keyword = scale or tally or zero
        scale values = type ratio
          type = atom type (1-N)
          ratio = factor by which to scale the damping coefficient
        tally values = no or yes
          no = do not tally the energy added/subtracted to atoms
          yes = do tally the energy added/subtracted to atoms

      zero value = no or yes
        no = do not set total random force to zero
        yes = set total random force to zero

## Examples

    fix 3 boundary langevin/eff 1.0 1.0 10.0 699483
    fix 1 all langevin/eff 1.0 1.1 10.0 48279 scale 3 1.5

## Description

Apply a Langevin thermostat as described in (Schneider) to a group of nuclei and electrons in the electron force field model. Used with fix nve/eff, this command performs Brownian dynamics (BD), since the total force on each atom will have the form:

\[\begin{split}F = & F_c + F_f + F_r \\ F_f = & - \frac{m}{\mathrm{damp}} v \\ F_r \propto & \sqrt{\frac{k_B T m}{dt~\mathrm{damp}}}\end{split}\]

\(F_c\) is the conservative force computed via the usual inter-particle interactions (pair_style). The \(F_f\) and \(F_r\) terms are added by this fix on a per-particle basis.

The operation of this fix is exactly like that described by the fix langevin command, except that the thermostatting is also applied to the radial electron velocity for electron particles.

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

No information about this fix is written to binary restart files. Because the state of the random number generator is not saved in restart files, this means you cannot do “exact” restarts with this fix, where the simulation continues on the same as if no restart had taken place. However, in a statistical sense, a restarted simulation should produce the same behavior.

The fix_modify temp option is supported by this fix. You can use it to assign a temperature compute you have defined to this fix which will be used in its thermostatting procedure, as described above. For consistency, the group used by this fix and by the compute should be the same.

The cumulative energy change in the system imposed by this fix is included in the thermodynamic output keywords ecouple and econserve, but only if the tally keyword to set to yes. See the thermo_style page for details.

This fix computes a global scalar which can be accessed by various output commands. The scalar is the same cumulative energy change due to this fix described in the previous paragraph. The scalar value calculated by this fix is “extensive”. Note that calculation of this quantity also requires setting the tally keyword to yes.

This fix can ramp its target temperature over multiple runs, using the start and stop keywords of the run command. See the run command for details of how to do this.

This fix is not invoked during energy minimization.

## Restrictions

none

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

## Related commands

fix langevin

## Default

The option defaults are scale = 1.0 for all types and tally = no.

(Dunweg) Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).

(Schneider) Schneider and Stoll, Phys Rev B, 17, 1302 (1978).