forked from lijiext/lammps
187 lines
8.3 KiB
HTML
187 lines
8.3 KiB
HTML
<HTML>
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<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>
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<H3>fix langevin command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID langevin Tstart Tstop damp seed keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>langevin = style name of this fix command
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<LI>Tstart,Tstop = desired temperature at start/end of run (temperature units)
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<LI>damp = damping parameter (time units)
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<LI>seed = random # seed to use for white noise (positive integer)
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<LI>zero or more keyword/value pairs may be appended
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<PRE>keyword = <I>scale</I>
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<I>scale</I> values = type ratio
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type = atom type (1-N)
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ratio = factor by which to scale the damping coefficient
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 3 boundary langevin 1.0 1.0 1000.0 699483
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fix 1 all langevin 1.0 1.1 100.0 48279 scale 3 1.5
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Apply a Langevin thermostat to a group of atoms which models an
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interaction with a background implicit solvent. Used with <A HREF = "fix_nve.html">fix
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nve</A>, this command performs Brownian dynamics (BD), since
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the total force on each atom will have the form:
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</P>
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<PRE>F = Fc + Ff + Fr
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Ff = - (m / damp) v
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Fr is proportional to sqrt(Kb T m / (dt damp))
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</PRE>
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<P>Fc is the conservative force computed via the usual inter-particle
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interactions (<A HREF = "pair_style.html">pair_style</A>,
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<A HREF = "bond_style.html">bond_style</A>, etc).
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</P>
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<P>The Ff and Fr terms are added by this fix.
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</P>
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<P>Ff is a frictional drag or viscous damping term proportional to the
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particle's velocity. The proportionality constant for each atom is
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computed as m/damp, where m is the mass of the particle and damp is
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the damping factor specified by the user.
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</P>
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<P>Fr is a force due to solvent atoms at a temperature T randomly bumping
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into the particle. As derived from the fluctuation/dissipation
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theorem, its magnitude as shown above is proportional to sqrt(Kb T m /
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dt damp), where Kb is the Boltzmann constant, T is the desired
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temperature, m is the mass of the particle, dt is the timestep size,
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and damp is the damping factor. Random numbers are used to randomize
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the direction and magnitude of this force as described in
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<A HREF = "#Dunweg">(Dunweg)</A>, where a uniform random number is used (instead of
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a Gaussian random number) for speed.
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</P>
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<P>Note that the thermostat effect of this fix is applied to only the
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translational degrees of freedom for the particles, which is an
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important consideration if extended spherical or aspherical particles
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which have rotational degrees of freedom are being thermostatted with
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this fix. The translational degrees of freedom can also have a bias
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velocity removed from them before thermostatting takes place; see the
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description below.
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</P>
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<P>IMPORTANT NOTE: Unlike the <A HREF = "fix_nvt.html">fix nvt</A> command which
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performs Nose/Hoover thermostatting AND time integration, this fix
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does NOT perform time integration. It only modifies forces to effect
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thermostatting. Thus you must use a separate time integration fix,
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like <A HREF = "fix_nve.html">fix nve</A> to actually update the velocities and
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positions of atoms using the modified forces. Likewise, this fix
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should not normally be used on atoms that also have their temperature
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controlled by another fix - e.g. by <A HREF = "fix_nvt.html">fix nvt</A> or <A HREF = "fix_temp_rescale.html">fix
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temp/rescale</A> commands.
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</P>
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<P>See <A HREF = "Section_howto.html#4_16">this howto section</A> of the manual for a
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discussion of different ways to compute temperature and perform
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thermostatting.
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</P>
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<P>The desired temperature at each timestep is a ramped value during the
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run from <I>Tstart</I> to <I>Tstop</I>.
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</P>
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<P>Like other fixes that perform thermostatting, this fix can be used
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with <A HREF = "compute.html">compute commands</A> that calculate a temperature
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after removing a "bias" from the atom velocities. E.g. removing the
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center-of-mass velocity from a group of atoms or only calculating
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temperature on the x-component of velocity or only calculating
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temperature for atoms in a geometric region. This is not done by
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default, but only if the <A HREF = "fix_modify.html">fix_modify</A> command is used
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to assign a temperature compute to this fix that includes such a bias
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term. See the doc pages for individual <A HREF = "compute.html">compute
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commands</A> to determine which ones include a bias. In
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this case, the thermostat works in the following manner: the current
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temperature is calculated taking the bias into account, bias is
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removed from each atom, thermostatting is performed on the remaining
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thermal degrees of freedom, and the bias is added back in.
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</P>
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<P>The <I>damp</I> parameter is specified in time units and determines how
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rapidly the temperature is relaxed. For example, a value of 100.0
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means to relax the temperature in a timespan of (roughly) 100 time
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units (tau or fmsec or psec - see the <A HREF = "units.html">units</A> command).
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The damp factor can be thought of as inversely related to the
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viscosity of the solvent. I.e. a small relaxation time implies a
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hi-viscosity solvent and vice versa. See the discussion about gamma
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and viscosity in the documentation for the <A HREF = "fix_viscous.html">fix
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viscous</A> command for more details.
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</P>
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<P>The random # <I>seed</I> must be a positive integer. A Marsaglia random
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number generator is used. Each processor uses the input seed to
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generate its own unique seed and its own stream of random numbers.
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Thus the dynamics of the system will not be identical on two runs on
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different numbers of processors.
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</P>
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<P>The keyword <I>scale</I> allows the damp factor to be scaled up or down by
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the specified factor for atoms of that type. This can be useful when
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different atom types have different sizes or masses. It can be used
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multiple times to adjust damp for several atom types. Note that
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specifying a ratio of 2 increases the relaxation time which is
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equivalent to the solvent's viscosity acting on particles with 1/2 the
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diameter. This is the opposite effect of scale factors used by the
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<A HREF = "fix_viscous.html">fix viscous</A> command, since the damp factor in fix
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<I>langevin</I> is inversely related to the gamma factor in fix <I>viscous</I>.
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Also note that the damping factor in fix <I>langevin</I> includes the
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particle mass in Ff, unlike fix <I>viscous</I>. Thus the mass and size of
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different atom types should be accounted for in the choice of ratio
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values.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. Because the state of the random number generator
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is not saved in restart files, this means you cannot do "exact"
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restarts with this fix, where the simulation continues on the same as
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if no restart had taken place. However, in a statistical sense, a
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restarted simulation should produce the same behavior.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>temp</I> option is supported by this
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fix. You can use it to assign a temperature <A HREF = "compute.html">compute</A>
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you have defined to this fix which will be used in its thermostatting
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procedure, as described above. For consistency, the group used by
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this fix and by the compute should be the same.
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</P>
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<P>No global scalar or vector or per-atom quantities are stored by this
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fix for access by various <A HREF = "Section_howto.html#4_15">output commands</A>.
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</P>
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<P>This fix can ramp its target temperature over multiple runs, using the
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<I>start</I> and <I>stop</I> keywords of the <A HREF = "run.html">run</A> command. See the
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<A HREF = "run.html">run</A> command for details of how to do this.
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</P>
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<P>This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_nvt.html">fix nvt</A>, <A HREF = "fix_temp_rescale.html">fix temp/rescale</A>, <A HREF = "fix_viscous.html">fix
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viscous</A>, <A HREF = "fix_nvt.html">fix nvt</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are axes = 1 1 1, scale = 1.0 for all types, no
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region, and weight = 1.0.
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</P>
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<HR>
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<A NAME = "Dunweg"></A>
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<P><B>(Dunweg)</B> Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).
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</P>
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</HTML>
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