forked from lijiext/lammps
150 lines
6.4 KiB
HTML
150 lines
6.4 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>axes</I> or <I>scale</I> or <I>region</I>
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<I>axes</I> values = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include a dimension in the thermostat
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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 to scale the damping coefficient by
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<I>region</I> values = region-ID
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region-ID = ID of region to apply thermostat to
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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 axes 0 1 1
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fix 3 boundary langevin 1.0 1.0 1000.0 699483 region boundary
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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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</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. Ff = - gamma v and is a
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frictional drag or viscous damping term proportional to the particle's
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velocity. Gamma for each atom is computed as m/damp, where m is the
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mass of the particle and damp is the damping factor specified by the
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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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theorum, its magnitude is proportional to sqrt(T m / dt damp), where T
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is the desired temperature, m is the mass of the particle, dt is the
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timestep size, and damp is the damping factor. Random numbers are
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used to randomize the direction and magnitude of this force.
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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>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>axes</I> can be used to specify which dimensions to add Ff
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and Fr to. A flag of 0 means skip that dimension; a flag of 1 means
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include that dimension. The default is 1 for all 3 dimensions.
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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. It can be used multiple
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times to adjust damp for several atom types. Note that specifying a
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ratio of 2 increase the relaxation time which is equivalent to the the
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solvent's viscosity acting on particles with 1/2 the diameter. This
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is the opposite effect of scale factors used by the <A HREF = "fix_viscous.html">fix
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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>The keyword <I>region</I> applies the fix only to atoms that are in the
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specified geometric region (and in the fix group). Since atoms can
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enter/leave a region, this test is performed each timestep.
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</P>
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<P>As noted above, fix <I>langevin</I> does not update the coordinates or
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velocities of its atoms, only the forces. It is normally used with a
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<A HREF = "fix_nve.html">fix nve</A> that does the time integration. Fix <I>langevin</I>
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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. a <A HREF = "fix_nvt.html">nvt</A> or
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<A HREF = "fix_temp_rescale.html">temp/rescale</A> fix.
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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>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix. No global scalar or vector or per-atom quantities are stored by
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this fix for access by various <A HREF = "Section_howto.html#4_15">output
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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>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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