forked from lijiext/lammps
129 lines
4.7 KiB
HTML
129 lines
4.7 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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<HR>
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<H3>fix langevin/eff 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/eff 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/eff = 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 number 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> or <I>tally</I> or <I>zero</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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<I>tally</I> values = <I>no</I> or <I>yes</I>
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<I>no</I> = do not tally the energy added/subtracted to atoms
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<I>yes</I> = do tally the energy added/subtracted to atoms
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</PRE>
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<PRE> <I>zero</I> value = <I>no</I> or <I>yes</I>
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<I>no</I> = do not set total random force to zero
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<I>yes</I> = set total random force to zero
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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/eff 1.0 1.0 10.0 699483
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fix 1 all langevin/eff 1.0 1.1 10.0 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 as described in <A HREF = "#Schneider">(Schneider)</A>
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to a group of nuclei and electrons in the <A HREF = "pair_eff.html">electron force
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field</A> model. Used with <A HREF = "fix_nve_eff.html">fix nve/eff</A>,
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this command performs Brownian dynamics (BD), since the total force on
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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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</P>
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<P>The Ff and Fr terms are added by this fix on a per-particle basis.
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</P>
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<P>The operation of this fix is exactly like that described by the <A HREF = "fix_langevin.html">fix
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langevin</A> command, except that the thermostatting
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is also applied to the radial electron velocity for electron
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particles.
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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>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the energy change induced by Langevin thermostatting to the
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system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>. Note that use of this option requires
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setting the <I>tally</I> keyword to <I>yes</I>.
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</P>
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<P>This fix computes a global scalar which can be accessed by various
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<A HREF = "Section_howto.html#howto_15">output commands</A>. The scalar is the
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cummulative energy change due to this fix. The scalar value
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calculated by this fix is "extensive". Note that calculation of this
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quantity requires setting the <I>tally</I> keyword to <I>yes</I>.
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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>This fix is part of the USER-EFF package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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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_langevin.html">fix langevin</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 scale = 1.0 for all types and tally = no.
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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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<A NAME = "Schneider"></A>
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<P><B>(Schneider)</B> Schneider and Stoll, Phys Rev B, 17, 1302 (1978).
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</P>
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</HTML>
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