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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 nvt/sphere command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID nvt/sphere Tstart Tstop Tdamp keyword value ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>nvt/sphere = 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/sphere 300.0 300.0 100.0
fix 1 all nvt/sphere 300.0 300.0 100.0 drag 0.2
</PRE>
<P><B>Description:</B>
</P>
<P>Perform constant NVT integration to update position, velocity, and
angular velocity each timestep for extended spherical particles 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>This fix differs from the <A HREF = "fix_nvt.html">fix nvt</A> command, which
assumes point particles and only updates their position and velocity.
</P>
<P>The thermostat is applied to both the translational and rotational
degrees of freedom for the spherical particles, assuming a compute is
used which calculates a temperature that includes the rotational
degrees of freedom (see below). The translational degrees of freedom
can also have a bias velocity removed from them before thermostatting
takes place; see the description below.
</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/sphere", as if this command
had been issued:
</P>
<PRE>compute fix-ID_temp group-ID temp/sphere
</PRE>
<P>See the <A HREF = "compute_temp_sphere.html">compute temp/sphere</A> command for
details. Note that the ID of the new compute is the fix-ID +
underscore + "temp", 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 thermodynamic 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>Like other fixes that perform thermostatting, this fix can be used
with <A HREF = "compute.html">compute commands</A> that calculate a temperature
after removing a "bias" from the atom velocities. E.g. removing the
center-of-mass velocity from a group of atoms or only calculating
temperature on the x-component of velocity or only calculating
temperature for atoms in a geometric region. This is not done by
default, but only if the <A HREF = "fix_modify.html">fix_modify</A> command is used
to assign a temperature compute to this fix that includes such a bias
term. See the doc pages for individual <A HREF = "compute.html">compute
commands</A> to determine which ones include a bias. In
this case, the thermostat works in the following manner: the current
temperature is calculated taking the bias into account, bias is
removed from each atom, thermostatting is performed on the remaining
thermal degrees of freedom, and the bias is added back in.
</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 cummulative 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>This fix requires that particles be represented as extended spheres
and not point particles. This means they will have an angular
velocity and a diameter which is determined by the <A HREF = "shape.html">shape</A>
command.
</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_nvt.html">fix nvt</A>, <A HREF = "fix_nve_sphere.html">fix nve_sphere</A>, <A HREF = "fix_nvt_asphere.html">fix
nvt_asphere</A>, <A HREF = "fix_npt_sphere.html">fix
npt_sphere</A>, <A HREF = "fix_modify.html">fix_modify</A>
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are drag = 0.0.
</P>
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