"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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fix heat command :h3

[Syntax:]

fix ID group-ID heat N eflux :pre

ID, group-ID are documented in "fix"_fix.html command
heat = style name of this fix command
N = add/subtract heat every this many timesteps
eflux = rate of heat addition or subtraction (energy/time units) :ul

[Examples:]

fix 3 qin heat 1 1.0
fix 4 qout heat 1 -1.0 :pre

[Description:]

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.

Heat addition/subtraction is performed every N timesteps.  The {eflux}
parameter determines the change in aggregate energy of the entire
group of atoms per unit time, e.g. in eV/psec for "metal
units"_units.html.  Thus it is an "extensive" quantity, meaning its
magnitude should be scaled with the number of atoms in the group.
Since {eflux} is independent of N or the "timestep"_timestep.html, 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.

Fix heat is different from a thermostat such as "fix nvt"_fix_nvt.html
or "fix temp/rescale"_fix_temp_rescale.html 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 "fix viscous"_fix_viscous to drain energy
from the system.

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. "fix nve"_fix_nve.html) on the affected atoms.  This fix should
not normally be used on atoms that have their temperature controlled
by another fix - e.g. "fix nvt"_fix_nvt.html or "fix
langevin"_fix_langevin.html fix.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  None of the "fix_modify"_fix_modify.html options
are relevant to this fix.

This fix computes a scalar which can be accessed by various "output
commands"_Section_howto.html#4_15.  This scalar is the most recent
value by which velocites were scaled.  The scalar value calculated by
this fix is "intensive".

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[Restrictions:] none

[Related commands:]

"compute temp"_compute_temp.html, "compute
temp/region"_compute_temp_region.html

[Default:] none

