lammps/doc/fix_langevin.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

fix langevin command :h3

[Syntax:]

fix ID group-ID langevin Tstart Tstop damp seed keyword values ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
langevin = style name of this fix command :l
Tstart,Tstop = desired temperature at start/end of run (temperature units) :l
damp = damping parameter (time units) :l
seed = random # seed to use for white noise (positive integer) :l

zero or more keyword/value pairs may be appended :l
keyword = {scale}
  {scale} values = type ratio
    type = atom type (1-N)
    ratio = factor by which to scale the damping coefficient :pre
:ule

[Examples:]

fix 3 boundary langevin 1.0 1.0 1000.0 699483
fix 1 all langevin 1.0 1.1 100.0 48279 scale 3 1.5 :pre

[Description:]

Apply a Langevin thermostat to a group of atoms which models an
interaction with a background implicit solvent.  Used with "fix
nve"_fix_nve.html, 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)) :pre

Fc is the conservative force computed via the usual inter-particle
interactions ("pair_style"_pair_style.html,
"bond_style"_bond_style.html, etc).

The Ff and Fr terms are added by this fix.

Ff is a frictional drag or viscous damping term proportional to the
particle's velocity.  The proportionality constant for each atom is
computed as m/damp, where m is the mass of the particle and damp is
the damping factor specified by the user.

Fr is a force due to solvent atoms at a temperature T randomly bumping
into the particle.  As derived from the fluctuation/dissipation
theorem, its magnitude as shown above is proportional to sqrt(Kb T m /
dt damp), where Kb is the Boltzmann constant, T is the desired
temperature, m is the mass of the particle, dt is the timestep size,
and damp is the damping factor.  Random numbers are used to randomize
the direction and magnitude of this force as described in
"(Dunweg)"_#Dunweg, where a uniform random number is used (instead of
a Gaussian random number) for speed.

Note that the thermostat effect of this fix is applied to only the
translational degrees of freedom for the particles, which is an
important consideration if extended spherical or aspherical particles
which have rotational degrees of freedom are being thermostatted with
this fix.  The translational degrees of freedom can also have a bias
velocity removed from them before thermostatting takes place; see the
description below.

IMPORTANT NOTE: Unlike the "fix nvt"_fix_nvt.html command which
performs Nose/Hoover thermostatting AND time integration, this fix
does NOT perform time integration.  It only modifies forces to effect
thermostatting.  Thus you must use a separate time integration fix,
like "fix nve"_fix_nve.html to actually update the velocities and
positions of atoms using the modified forces.  Likewise, this fix
should not normally be used on atoms that also have their temperature
controlled by another fix - e.g. by "fix nvt"_fix_nvt.html or "fix
temp/rescale"_fix_temp_rescale.html commands.

See "this howto section"_Section_howto.html#4_16 of the manual for a
discussion of different ways to compute temperature and perform
thermostatting.

The desired temperature at each timestep is a ramped value during the
run from {Tstart} to {Tstop}.

Like other fixes that perform thermostatting, this fix can be used
with "compute commands"_compute.html that calculate a temperature
after removing a "bias" from the atom velocities.  E.g. removing the
center-of-mass velocity from a group of atoms or only calculating
temperature on the x-component of velocity or only calculating
temperature for atoms in a geometric region.  This is not done by
default, but only if the "fix_modify"_fix_modify.html command is used
to assign a temperature compute to this fix that includes such a bias
term.  See the doc pages for individual "compute
commands"_compute.html to determine which ones include a bias.  In
this case, the thermostat works in the following manner: the current
temperature is calculated taking the bias into account, bias is
removed from each atom, thermostatting is performed on the remaining
thermal degrees of freedom, and the bias is added back in.

The {damp} parameter is specified in time units and determines how
rapidly the temperature is relaxed.  For example, a value of 100.0
means to relax the temperature in a timespan of (roughly) 100 time
units (tau or fmsec or psec - see the "units"_units.html command).
The damp factor can be thought of as inversely related to the
viscosity of the solvent.  I.e. a small relaxation time implies a
hi-viscosity solvent and vice versa.  See the discussion about gamma
and viscosity in the documentation for the "fix
viscous"_fix_viscous.html command for more details.

The random # {seed} must be a positive integer.  A Marsaglia random
number generator is used.  Each processor uses the input seed to
generate its own unique seed and its own stream of random numbers.
Thus the dynamics of the system will not be identical on two runs on
different numbers of processors.

The keyword {scale} allows the damp factor to be scaled up or down by
the specified factor for atoms of that type.  This can be useful when
different atom types have different sizes or masses.  It can be used
multiple times to adjust damp for several atom types.  Note that
specifying a ratio of 2 increases the relaxation time which is
equivalent to the solvent's viscosity acting on particles with 1/2 the
diameter.  This is the opposite effect of scale factors used by the
"fix viscous"_fix_viscous.html command, since the damp factor in fix
{langevin} is inversely related to the gamma factor in fix {viscous}.
Also note that the damping factor in fix {langevin} includes the
particle mass in Ff, unlike fix {viscous}.  Thus the mass and size of
different atom types should be accounted for in the choice of ratio
values.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  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.html {temp} option is supported by this
fix.  You can use it to assign a temperature "compute"_compute.html
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.

No global scalar or vector or per-atom quantities are stored by this
fix for access by various "output commands"_Section_howto.html#4_15.

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

This fix is not invoked during "energy minimization"_minimize.html.

[Restrictions:] none

[Related commands:]

"fix nvt"_fix_nvt.html, "fix temp/rescale"_fix_temp_rescale.html, "fix
viscous"_fix_viscous.html, "fix nvt"_fix_nvt.html

[Default:]

The option defaults are axes = 1 1 1, scale = 1.0 for all types, no
region, and weight = 1.0.

:line

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