forked from lijiext/lammps
363 lines
17 KiB
HTML
363 lines
17 KiB
HTML
|
<HTML>
|
||
|
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
|
||
|
</CENTER>
|
||
|
|
||
|
|
||
|
|
||
|
|
||
|
|
||
|
|
||
|
<HR>
|
||
|
|
||
|
<H3>fix langevin command
|
||
|
</H3>
|
||
|
<H3>fix langevin/kk command
|
||
|
</H3>
|
||
|
<P><B>Syntax:</B>
|
||
|
</P>
|
||
|
<PRE>fix ID group-ID langevin Tstart Tstop damp seed keyword values ...
|
||
|
</PRE>
|
||
|
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
|
||
|
|
||
|
<LI>langevin = style name of this fix command
|
||
|
|
||
|
<LI>Tstart,Tstop = desired temperature at start/end of run (temperature units)
|
||
|
|
||
|
<LI>Tstart can be a variable (see below)
|
||
|
|
||
|
<LI>damp = damping parameter (time units)
|
||
|
|
||
|
<LI>seed = random number seed to use for white noise (positive integer)
|
||
|
|
||
|
<LI>zero or more keyword/value pairs may be appended
|
||
|
|
||
|
<LI>keyword = <I>angmom</I> or <I>omega</I> or <I>scale</I> or <I>tally</I> or <I>zero</I>
|
||
|
|
||
|
<PRE> <I>angmom</I> value = <I>no</I> or scale
|
||
|
<I>no</I> = do not thermostat rotational degrees of freedom via the angular momentum
|
||
|
factor = do thermostat rotational degrees of freedom via the angular momentum and apply numeric factor as discussed below
|
||
|
<I>gjf</I> value = <I>no</I> or <I>yes</I>
|
||
|
<I>no</I> = use standard formulation
|
||
|
<I>yes</I> = use Gronbech-Jensen/Farago formulation
|
||
|
<I>omega</I> value = <I>no</I> or <I>yes</I>
|
||
|
<I>no</I> = do not thermostat rotational degrees of freedom via the angular velocity
|
||
|
<I>yes</I> = do thermostat rotational degrees of freedom via the angular velocity
|
||
|
<I>scale</I> values = type ratio
|
||
|
type = atom type (1-N)
|
||
|
ratio = factor by which to scale the damping coefficient
|
||
|
<I>tally</I> value = <I>no</I> or <I>yes</I>
|
||
|
<I>no</I> = do not tally the energy added/subtracted to atoms
|
||
|
<I>yes</I> = do tally the energy added/subtracted to atoms
|
||
|
<I>zero</I> value = <I>no</I> or <I>yes</I>
|
||
|
<I>no</I> = do not set total random force to zero
|
||
|
<I>yes</I> = set total random force to zero
|
||
|
</PRE>
|
||
|
|
||
|
</UL>
|
||
|
<P><B>Examples:</B>
|
||
|
</P>
|
||
|
<PRE>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
|
||
|
fix 1 all langevin 1.0 1.1 100.0 48279 angmom 3.333
|
||
|
</PRE>
|
||
|
<P><B>Description:</B>
|
||
|
</P>
|
||
|
<P>Apply a Langevin thermostat as described in <A HREF = "#Schneider">(Schneider)</A>
|
||
|
to a group of atoms which models an interaction with a background
|
||
|
implicit solvent. Used with <A HREF = "fix_nve.html">fix nve</A>, this command
|
||
|
performs Brownian dynamics (BD), since the total force on each atom
|
||
|
will have the form:
|
||
|
</P>
|
||
|
<PRE>F = Fc + Ff + Fr
|
||
|
Ff = - (m / damp) v
|
||
|
Fr is proportional to sqrt(Kb T m / (dt damp))
|
||
|
</PRE>
|
||
|
<P>Fc is the conservative force computed via the usual inter-particle
|
||
|
interactions (<A HREF = "pair_style.html">pair_style</A>,
|
||
|
<A HREF = "bond_style.html">bond_style</A>, etc).
|
||
|
</P>
|
||
|
<P>The Ff and Fr terms are added by this fix on a per-particle basis.
|
||
|
See the <A HREF = "pair_dpd.html">pair_style dpd/tstat</A> command for a
|
||
|
thermostatting option that adds similar terms on a pairwise basis to
|
||
|
pairs of interacting particles.
|
||
|
</P>
|
||
|
<P>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.
|
||
|
</P>
|
||
|
<P>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
|
||
|
<A HREF = "#Dunweg">(Dunweg)</A>, where a uniform random number is used (instead of
|
||
|
a Gaussian random number) for speed.
|
||
|
</P>
|
||
|
<P>Note that unless you use the <I>omega</I> or <I>angmom</I> keywords, the
|
||
|
thermostat effect of this fix is applied to only the translational
|
||
|
degrees of freedom for the particles, which is an important
|
||
|
consideration for finite-size particles, which have rotational degrees
|
||
|
of freedom, are being thermostatted. The translational degrees of
|
||
|
freedom can also have a bias velocity removed from them before
|
||
|
thermostatting takes place; see the description below.
|
||
|
</P>
|
||
|
<P>IMPORTANT NOTE: Unlike the <A HREF = "fix_nh.html">fix nvt</A> 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 <A HREF = "fix_nve.html">fix nve</A> 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 <A HREF = "fix_nh.html">fix nvt</A> or <A HREF = "fix_temp_rescale.html">fix
|
||
|
temp/rescale</A> commands.
|
||
|
</P>
|
||
|
<P>See <A HREF = "Section_howto.html#howto_16">this howto section</A> of the manual for
|
||
|
a discussion of different ways to compute temperature and perform
|
||
|
thermostatting.
|
||
|
</P>
|
||
|
<P>The desired temperature at each timestep is a ramped value during the
|
||
|
run from <I>Tstart</I> to <I>Tstop</I>.
|
||
|
</P>
|
||
|
<P><I>Tstart</I> can be specified as an equal-style or atom-style
|
||
|
<A HREF = "variable.html">variable</A>. In this case, the <I>Tstop</I> setting is
|
||
|
ignored. If the value is a variable, it should be specified as
|
||
|
v_name, where name is the variable name. In this case, the variable
|
||
|
will be evaluated each timestep, and its value used to determine the
|
||
|
target temperature.
|
||
|
</P>
|
||
|
<P>Equal-style variables can specify formulas with various mathematical
|
||
|
functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
|
||
|
keywords for the simulation box parameters and timestep and elapsed
|
||
|
time. Thus it is easy to specify a time-dependent temperature.
|
||
|
</P>
|
||
|
<P>Atom-style variables can specify the same formulas as equal-style
|
||
|
variables but can also include per-atom values, such as atom
|
||
|
coordinates. Thus it is easy to specify a spatially-dependent
|
||
|
temperature with optional time-dependence as well.
|
||
|
</P>
|
||
|
<P>Like other fixes that perform thermostatting, this fix can be used
|
||
|
with <A HREF = "compute.html">compute commands</A> that remove a "bias" from the
|
||
|
atom velocities. E.g. removing the center-of-mass velocity from a
|
||
|
group of atoms or removing the x-component of velocity from the
|
||
|
calculation. This is not done by default, but only if the
|
||
|
<A HREF = "fix_modify.html">fix_modify</A> command is used to assign a temperature
|
||
|
compute to this fix that includes such a bias term. See the doc pages
|
||
|
for individual <A HREF = "compute.html">compute commands</A> to determine which ones
|
||
|
include a bias. In this case, the thermostat works in the following
|
||
|
manner: bias is removed from each atom, thermostatting is performed on
|
||
|
the remaining thermal degrees of freedom, and the bias is added back
|
||
|
in.
|
||
|
</P>
|
||
|
<P>The <I>damp</I> 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 <A HREF = "units.html">units</A> 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 <A HREF = "fix_viscous.html">fix
|
||
|
viscous</A> command for more details.
|
||
|
</P>
|
||
|
<P>The random # <I>seed</I> 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.
|
||
|
</P>
|
||
|
<HR>
|
||
|
|
||
|
<P>The keyword/value option pairs are used in the following ways.
|
||
|
</P>
|
||
|
<P>The keyword <I>angmom</I> and <I>omega</I> keywords enable thermostatting of
|
||
|
rotational degrees of freedom in addition to the usual translational
|
||
|
degrees of freedom. This can only be done for finite-size particles.
|
||
|
</P>
|
||
|
<P>A simulation using atom_style sphere defines an omega for finite-size
|
||
|
spheres. A simulation using atom_style ellipsoid defines a finite
|
||
|
size and shape for aspherical particles and an angular momentum.
|
||
|
The Langevin formulas for thermostatting the rotational degrees of
|
||
|
freedom are the same as those above, where force is replaced by
|
||
|
torque, m is replaced by the moment of inertia I, and v is replaced by
|
||
|
omega (which is derived from the angular momentum in the case of
|
||
|
aspherical particles).
|
||
|
</P>
|
||
|
<P>The rotational temperature of the particles can be monitored by the
|
||
|
<A HREF = "compute_temp_sphere.html">compute temp/sphere</A> and <A HREF = "compute_temp_asphere.html">compute
|
||
|
temp/asphere</A> commands with their rotate
|
||
|
options.
|
||
|
</P>
|
||
|
<P>For the <I>omega</I> keyword there is also a scale factor of 10.0/3.0 that
|
||
|
is applied as a multiplier on the Ff (damping) term in the equation
|
||
|
above and of sqrt(10.0/3.0) as a multiplier on the Fr term. This does
|
||
|
not affect the thermostatting behaviour of the Langevin formalism but
|
||
|
insures that the randomized rotational diffusivity of spherical
|
||
|
particles is correct.
|
||
|
</P>
|
||
|
<P>For the <I>angmom</I> keyword a similar scale factor is needed which is
|
||
|
10.0/3.0 for spherical particles, but is anisotropic for aspherical
|
||
|
particles (e.g. ellipsoids). Currently LAMMPS only applies an
|
||
|
isotropic scale factor, and you can choose its magnitude as the
|
||
|
specified value of the <I>angmom</I> keyword. If your aspherical particles
|
||
|
are (nearly) spherical than a value of 10.0/3.0 = 3.333 is a good
|
||
|
choice. If they are highly aspherical, a value of 1.0 is as good a
|
||
|
choice as any, since the effects on rotational diffusivity of the
|
||
|
particles will be incorrect regardless. Note that for any reasonable
|
||
|
scale factor, the thermostatting effect of the <I>angmom</I> keyword on the
|
||
|
rotational temperature of the aspherical particles should still be
|
||
|
valid.
|
||
|
</P>
|
||
|
<P>The keyword <I>scale</I> 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
|
||
|
<A HREF = "fix_viscous.html">fix viscous</A> command, since the damp factor in fix
|
||
|
<I>langevin</I> is inversely related to the gamma factor in fix <I>viscous</I>.
|
||
|
Also note that the damping factor in fix <I>langevin</I> includes the
|
||
|
particle mass in Ff, unlike fix <I>viscous</I>. Thus the mass and size of
|
||
|
different atom types should be accounted for in the choice of ratio
|
||
|
values.
|
||
|
</P>
|
||
|
<P>The keyword <I>tally</I> enables the calculation of the cumulative energy
|
||
|
added/subtracted to the atoms as they are thermostatted. Effectively
|
||
|
it is the energy exchanged between the infinite thermal reservoir and
|
||
|
the particles. As described below, this energy can then be printed
|
||
|
out or added to the potential energy of the system to monitor energy
|
||
|
conservation.
|
||
|
</P>
|
||
|
<P>IMPORTANT NOTE: this accumulated energy does NOT include kinetic
|
||
|
energy removed by the <I>zero</I> flag. LAMMPS will print a warning when
|
||
|
both options are active.
|
||
|
</P>
|
||
|
<P>The keyword <I>zero</I> can be used to eliminate drift due to the
|
||
|
thermostat. Because the random forces on different atoms are
|
||
|
independent, they do not sum exactly to zero. As a result, this fix
|
||
|
applies a small random force to the entire system, and the
|
||
|
center-of-mass of the system undergoes a slow random walk. If the
|
||
|
keyword <I>zero</I> is set to <I>yes</I>, the total random force is set exactly
|
||
|
to zero by subtracting off an equal part of it from each atom in the
|
||
|
group. As a result, the center-of-mass of a system with zero initial
|
||
|
momentum will not drift over time.
|
||
|
</P>
|
||
|
<P>The keyword <I>gjf</I> can be used to run the <A HREF = "#Gronbech-Jensen">Gronbech-Jensen/Farago
|
||
|
</A> time-discretization of the Langevin model. As
|
||
|
described in the papers cited below, the purpose of this method is to
|
||
|
enable longer timesteps to be used (up to the numerical stability
|
||
|
limit of the integrator), while still producing the correct Boltzmann
|
||
|
distribution of atom positions. It is implemented within LAMMPS, by
|
||
|
changing how the the random force is applied so that it is composed of
|
||
|
the average of two random forces representing half-contributions from
|
||
|
the previous and current time intervals.
|
||
|
</P>
|
||
|
<P>In common with all methods based on Verlet integration, the
|
||
|
discretized velocities generated by this method in conjunction with
|
||
|
velocity-Verlet time integration are not exactly conjugate to the
|
||
|
positions. As a result the temperature (computed from the discretized
|
||
|
velocities) will be systematically lower than the target temperature,
|
||
|
by a small amount which grows with the timestep. Nonetheless, the
|
||
|
distribution of atom positions will still be consistent with the
|
||
|
target temperature.
|
||
|
</P>
|
||
|
<P>As an example of using the <I>gjf</I> keyword, for molecules containing C-H
|
||
|
bonds, configurational properties generated with dt = 2.5 fs and tdamp
|
||
|
= 100 fs are indistinguishable from dt = 0.5 fs. Because the velocity
|
||
|
distribution systematically decreases with increasing timestep, the
|
||
|
method should not be used to generate properties that depend on the
|
||
|
velocity distribution, such as the velocity autocorrelation function
|
||
|
(VACF). In this example, the velocity distribution at dt = 2.5fs
|
||
|
generates an average temperature of 220 K, instead of 300 K.
|
||
|
</P>
|
||
|
<HR>
|
||
|
|
||
|
<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>intel</I>, <I>kk</I>, <I>omp</I>, or <I>opt</I> suffix are
|
||
|
functionally the same as the corresponding style without the suffix.
|
||
|
They have been optimized to run faster, depending on your available
|
||
|
hardware, as discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A>
|
||
|
of the manual. The accelerated styles take the same arguments and
|
||
|
should produce the same results, except for round-off and precision
|
||
|
issues.
|
||
|
</P>
|
||
|
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
|
||
|
KOKKOS, USER-OMP and OPT packages, respectively. They are only
|
||
|
enabled if LAMMPS was built with those packages. See the <A HREF = "Section_start.html#start_3">Making
|
||
|
LAMMPS</A> section for more info.
|
||
|
</P>
|
||
|
<P>You can specify the accelerated styles explicitly in your input script
|
||
|
by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
|
||
|
switch</A> when you invoke LAMMPS, or you can
|
||
|
use the <A HREF = "suffix.html">suffix</A> command in your input script.
|
||
|
</P>
|
||
|
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
|
||
|
more instructions on how to use the accelerated styles effectively.
|
||
|
</P>
|
||
|
<HR>
|
||
|
|
||
|
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
|
||
|
</P>
|
||
|
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
|
||
|
files</A>. 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.
|
||
|
</P>
|
||
|
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>temp</I> option is supported by this
|
||
|
fix. You can use it to assign a temperature <A HREF = "compute.html">compute</A>
|
||
|
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.
|
||
|
</P>
|
||
|
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
|
||
|
fix to add the energy change induced by Langevin thermostatting to the
|
||
|
system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
|
||
|
output</A>. Note that use of this option requires
|
||
|
setting the <I>tally</I> keyword to <I>yes</I>.
|
||
|
</P>
|
||
|
<P>This fix computes a global scalar which can be accessed by various
|
||
|
<A HREF = "Section_howto.html#howto_15">output commands</A>. The scalar is the
|
||
|
cummulative energy change due to this fix. The scalar value
|
||
|
calculated by this fix is "extensive". Note that calculation of this
|
||
|
quantity requires setting the <I>tally</I> keyword to <I>yes</I>.
|
||
|
</P>
|
||
|
<P>This fix can ramp its target temperature over multiple runs, using the
|
||
|
<I>start</I> and <I>stop</I> keywords of the <A HREF = "run.html">run</A> command. See the
|
||
|
<A HREF = "run.html">run</A> command for details of how to do this.
|
||
|
</P>
|
||
|
<P>This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
|
||
|
</P>
|
||
|
<P><B>Restrictions:</B> none
|
||
|
</P>
|
||
|
<P><B>Related commands:</B>
|
||
|
</P>
|
||
|
<P><A HREF = "fix_nh.html">fix nvt</A>, <A HREF = "fix_temp_rescale.html">fix temp/rescale</A>, <A HREF = "fix_viscous.html">fix
|
||
|
viscous</A>, <A HREF = "fix_nh.html">fix nvt</A>, <A HREF = "pair_dpd.html">pair_style
|
||
|
dpd/tstat</A>
|
||
|
</P>
|
||
|
<P><B>Default:</B>
|
||
|
</P>
|
||
|
<P>The option defaults are angmom = no, omega = no, scale = 1.0 for all
|
||
|
types, tally = no, zero = no, gjf = no.
|
||
|
</P>
|
||
|
<HR>
|
||
|
|
||
|
<A NAME = "Dunweg"></A>
|
||
|
|
||
|
<P><B>(Dunweg)</B> Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).
|
||
|
</P>
|
||
|
<A NAME = "Schneider"></A>
|
||
|
|
||
|
<P><B>(Schneider)</B> Schneider and Stoll, Phys Rev B, 17, 1302 (1978).
|
||
|
</P>
|
||
|
<A NAME = "Gronbech-Jensen"></A>
|
||
|
|
||
|
<P><B>(Gronbech-Jensen)</B> Gronbech-Jensen and Farago, Mol Phys, 111, 983
|
||
|
(2013); Gronbech-Jensen, Hayre, and Farago, Comp Phys Comm,
|
||
|
185, 524 (2014)
|
||
|
</P>
|
||
|
</HTML>
|