forked from lijiext/lammps
103 lines
4.0 KiB
HTML
103 lines
4.0 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 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 to the args
|
|
|
|
<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. 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 <A HREF = "run.html">run</A> command documents
|
|
how to make the ramping take place across multiple runs. 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. The fix creates its
|
|
own method for computing T, as if it had been defined by the command:
|
|
</P>
|
|
<PRE>temperature fix-ID group-ID full
|
|
</PRE>
|
|
<P>See the <A HREF = "temperature.html">temperature</A> command for details. Note that
|
|
this is NOT the temperature with ID = <I>default</I>. This means you can
|
|
change the attributes of this fix's temperature (e.g. its
|
|
degrees-of-freedom) via the <A HREF = "temp_modify.html">temp_modify</A> command or
|
|
print the temperature with thermodyanmic output via the <A HREF = "thermo_style.html">thermo_style
|
|
custom</A> command using the appropriate temp-ID =
|
|
fix-ID. It also means that changing attributes of the default
|
|
temperature will have no effect on this fix. Alternatively, you can
|
|
directly assign a new temperature to the fix via the
|
|
<A HREF = "fix_modify.html">fix_modify</A> command.
|
|
</P>
|
|
<P>This fix supports the <A HREF = "fix_modify.html">fix_modify</A> options for
|
|
<I>thermo</I> and <I>energy</I>. The former will print the contribution the fix
|
|
makes to the energy of the system when thermodynamics is printed. The
|
|
latter will add this contribution to the total potential energy
|
|
(PotEng) so that energy conservation can be monitored.
|
|
</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>
|
|
</HTML>
|