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fix_nvt.html
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<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 nvt command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID nvt Tstart Tstop Tdamp keyword value ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>nvt = style name of this fix command
<LI>Tstart,Tstop = desired temperature at start/end of run
<LI>Tdamp = temperature damping parameter (time units)
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>drag</I>
<PRE> <I>drag</I> value = drag factor added to thermostat (0.0 = no drag)
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all nvt 300.0 300.0 100.0
fix 1 all nvt 300.0 300.0 100.0 drag 0.2
</PRE>
<P><B>Description:</B>
</P>
<P>Perform constant NVT integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover temperature
thermostat <A HREF = "#Hoover">(Hoover)</A>. V is volume; T is temperature. This
creates a system trajectory consistent with the canonical ensemble.
</P>
<P>The desired temperature at each timestep is a ramped value during the
run from <I>Tstart</I> to <I>Tstop</I>. The <I>Tdamp</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).
</P>
<P>In some cases (e.g. for solids) the temperature of the system can
oscillate undesirably when a Nose/Hoover thermostat is applied. The
optional <I>drag</I> keyword will damp these oscillations, although it
alters the Nose/Hoover equations. A value of 0.0 (no drag) leaves the
Nose/Hoover formalism unchanged. A non-zero value adds a drag term;
the larger the value specified, the greater the damping effect.
Performing a short run and monitoring the temperature is the best way
to determine if the drag term is working. Typically a value between
0.2 to 2.0 is sufficient to damp oscillations after a few periods.
</P>
<P>This fix computes a temperature each timestep. To do this, the fix
creates its own compute of style "temp", as if this command had been
issued:
</P>
<PRE>compute fix-ID_temp group-ID temp
</PRE>
<P>See the <A HREF = "compute_temp.html<A HREF = "temp"">>compute temp</A> command for details. Note
that the ID of the new compute is the fix-ID with </A> appended and
the group for the new compute is the same as the fix group.
</P>
<P>Note that this is NOT the compute used by thermodynamic output (see
the <A HREF = "thermo_style.html">thermo_style</A> command) with ID = <I>thermo_temp</I>.
This means you can change the attributes of this fix's temperature
(e.g. its degrees-of-freedom) via the
<A HREF = "compute_modify.html">compute_modify</A> command or print this temperature
during thermodyanmic output via the <A HREF = "thermo_style.html">thermo_style
custom</A> command using the appropriate compute-ID.
It also means that changing attributes of <I>thermo_temp</I> will have no
effect on this fix.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>This fix writes the state of the Nose/Hoover thermostat to <A HREF = "restart.html">binary
restart files</A>. See the <A HREF = "read_restart.html">read_restart</A>
command for info on how to re-specify a fix in an input script that
reads a restart file, so that the operation of the fix continues in an
uninterrupted fashion.
</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 <A HREF = "compute.html">compute</A> you have
defined to this fix which will be used in its thermostatting
procedure.
</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 Nose/Hoover thermostatting to
the system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
output</A>.
</P>
<P>The potential energy change due to this fix is stored as a scalar
quantity, which can be accessed by various <A HREF = "Section_howto.html#4_15">output
commands</A>. The scalar value calculated by
this fix is "extensive", meaning it scales with the number of atoms in
the simulation.
</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>
</P>
<P>The final Tstop cannot be 0.0 since it would make the target T = 0.0
at some timestep during the simulation which is not allowed in
the Nose/Hoover formulation.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_npt.html">fix npt</A>, <A HREF = "fix_temp_rescale.html">fix
temp/rescale</A>, <A HREF = "fix_langevin.html">fix langevin</A>,
<A HREF = "fix_modify.html">fix_modify</A>, <A HREF = "temperature.html">temperature</A>
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are drag = 0.0.
</P>
<HR>
<A NAME = "Hoover"></A>
<P><B>(Hoover)</B> Hoover, Phys Rev A, 31, 1695 (1985).
</P>
</HTML>
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