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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)
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 3 qin heat 1 1.0
fix 4 qout heat 1 -1.0
</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 to one group of atoms (hot reservoir) and subtracting heat
from another (cold reservoir). E.g. a simulation sampling from the
McDLT ensemble. Note that the fix is applied to a group of atoms, not
a geometric region, so that the same set of atoms is 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. Since eflux is in units of energy/time, this means a
larger value of N will add/subtract a larger amount of energy each
timestep the fix is invoked. If heat is subtracted from the system
too aggressively so that the group's kinetic energy goes to zero,
LAMMPS halts with an error message.
</P>
<P>Fix heat is different from a thermostat such as <A HREF = "fix_nvt.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">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_nvt.html">fix nvt</A> or <A HREF = "fix_langevin.html">fix
langevin</A> fix.
</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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