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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>
</CENTER>
<HR>
<H3>fix heat command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID heat N eflux
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>heat = style name of this fix command
<LI>N = add/subtract heat every this many timesteps
<LI>eflux = rate of heat addition or subtraction (energy/time units)
<LI>zero or more keyword/value pairs may be appended to args
<LI>keyword = <I>region</I>
<PRE> <I>region</I> value = region-ID
region-ID = ID of region atoms must be in to have added force
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 3 qin heat 1 1.0
fix 4 qout heat 1 -1.0 region top
</PRE>
<P><B>Description:</B>
</P>
<P>Add non-translational kinetic energy (heat) to a group of atoms such
that their aggregate momentum is conserved. Two of these fixes can be
used to establish a temperature gradient across a simulation domain by
adding heat (energy) to one group of atoms (hot reservoir) and
subtracting heat from another (cold reservoir). E.g. a simulation
sampling from the McDLT ensemble.
</P>
<P>If the <I>region</I> keyword is used, the atom must be in both the group
and the specified geometric <A HREF = "region.html">region</A> in order to have
energy added or subtracted to it. If not specified, then the atoms in
the group are affected wherever they may move to.
</P>
<P>Heat addition/subtraction is performed every N timesteps. The <I>eflux</I>
parameter determines the change in aggregate energy of the entire
group of atoms per unit time, e.g. in eV/psec for <A HREF = "units.html">metal
units</A>. Thus it is an "extensive" quantity, meaning its
magnitude should be scaled with the number of atoms in the group.
Since <I>eflux</I> is independent of N or the <A HREF = "timestep.html">timestep</A>, a
larger value of N will add/subtract a larger amount of energy each
time the fix is invoked. If heat is subtracted from the system too
aggressively so that the group's kinetic energy would go to zero,
LAMMPS halts with an error message.
</P>
<P>Fix heat is different from a thermostat such as <A HREF = "fix_nh.html">fix nvt</A>
or <A HREF = "fix_temp_rescale.html">fix temp/rescale</A> in that energy is
added/subtracted continually. Thus if there isn't another mechanism
in place to counterbalance this effect, the entire system will heat or
cool continuously. You can use multiple heat fixes so that the net
energy change is 0.0 or use <A HREF = "fix_viscous.html">fix viscous</A> to drain
energy from the system.
</P>
<P>This fix does not change the coordinates of its atoms; it only scales
their velocities. Thus you must still use an integration fix
(e.g. <A HREF = "fix_nve.html">fix nve</A>) on the affected atoms. This fix should
not normally be used on atoms that have their temperature controlled
by another fix - e.g. <A HREF = "fix_nh.html">fix nvt</A> or <A HREF = "fix_langevin.html">fix
langevin</A> fix.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix.
</P>
<P>This fix computes a global scalar which can be accessed by various
<A HREF = "Section_howto.html#howto_15">output commands</A>. This scalar is the
most recent value by which velocites were scaled. The scalar value
calculated by this fix is "intensive".
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
minimization</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_temp.html">compute temp</A>, <A HREF = "compute_temp_region.html">compute
temp/region</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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