# 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:

F = Fc + Ff + Fr
Ff = - (m / damp) v
Fr is proportional to sqrt(Kb T m / (dt damp))


Fc is the conservative force computed via the usual inter-particle interactions (pair_style).

The Ff and Fr 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 fix_modify energy option is supported by this fix to add the energy change induced by Langevin thermostatting to the system’s potential energy as part of thermodynamic output. Note that use of this option requires setting the tally keyword to yes.

This fix computes a global scalar which can be accessed by various output commands. The scalar is the cumulative energy change due to this fix. The scalar value calculated by this fix is “extensive”. Note that calculation of this quantity 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 USER-EFF package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.

## 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).