# fix langevin/eff command

## Syntax

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

-   ID, group-ID are documented in [fix](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

``` LAMMPS
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)](Schneider2) to
a group of nuclei and electrons in the [electron force field](pair_eff)
model. Used with [fix nve/eff](fix_nve_eff), this command performs
Brownian dynamics (BD), since the total force on each atom will have the
form:

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

$F_c$ is the conservative force computed via the usual inter-particle
interactions ([pair_style](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](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](restart). 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](fix_modify) *temp* option is supported by this fix. You
can use it to assign a temperature [compute](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](thermo_style) keywords *ecouple*
and *econserve*, but only if the *tally* keyword to set to *yes*. See
the [thermo_style](thermo_style) page for details.

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). 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](run) command. See the
[run](run) command for details of how to do this.

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

## 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](Build_package) page for
more info.

## Related commands

[fix langevin](fix_langevin)

## Default

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

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

::: {#Dunweg2}
**(Dunweg)** Dunweg and Paul, Int J of Modern Physics C, 2, 817-27
(1991).
:::

::: {#Schneider2}
**(Schneider)** Schneider and Stoll, Phys Rev B, 17, 1302 (1978).
:::