# fix heat command

## Syntax

    fix ID group-ID heat N eflux

-   ID, group-ID are documented in [fix](fix) 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)

-   eflux can be a variable (see below)

-   zero or more keyword/value pairs may be appended to args

-   keyword = *region*

        *region* value = region-ID
          region-ID = ID of region atoms must be in to have added force

## Examples

``` LAMMPS
fix 3 qin heat 1 1.0
fix 3 qin heat 10 v_flux
fix 4 qout heat 1 -1.0 region top
```

## Description

Add non-translational kinetic energy (heat) to a group of atoms in a
manner that conserves their aggregate momentum. 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.

If the *region* keyword is used, the atom must be in both the group and
the specified geometric [region](region) 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.

Heat addition/subtraction is performed every N timesteps.

The *eflux* parameter can be specified as a numeric constant or as an
equal- or atom-style [variable](variable). If the value is a variable,
it should be specified as v_name, where *name* is the variable name. In
this case, the variable will be evaluated each timestep, and its current
value(s) used to determine the flux.

If *eflux* is a numeric constant or equal-style variable which evaluates
to a scalar value, then *eflux* determines the change in aggregate
energy of the entire group of atoms per unit time, e.g. in eV/ps for
[metal units](units). In this case it is an \"extensive\" quantity,
meaning its magnitude should be scaled with the number of atoms in the
group. Note that since *eflux* also has per-time units (i.e. it is a
flux), this means that a larger value of N will add/subtract a larger
amount of energy each time the fix is invoked.

:::: note
::: title
Note
:::

The heat-exchange (HEX) algorithm implemented by this fix is known to
exhibit a pronounced energy drift. An improved algorithm (eHEX) is
available as a [fix ehex](fix_ehex) command and might be preferable if
energy conservation is important.
::::

If *eflux* is specified as an atom-style variable (see below), then the
variable computes one value per atom. In this case, each value is the
energy flux for a single atom, again in units of energy per unit time.
In this case, each value is an \"intensive\" quantity, which need not be
scaled with the number of atoms in the group.

Equal-style variables can specify formulas with various mathematical
functions, and include [thermo_style](thermo_style) command keywords for
the simulation box parameters and timestep and elapsed time. Thus it is
easy to specify a time-dependent flux.

Atom-style variables can specify the same formulas as equal-style
variables but can also include per-atom values, such as atom
coordinates. Thus it is easy to specify a spatially-dependent flux with
optional time-dependence as well.

:::: note
::: title
Note
:::

If heat is subtracted from the system too aggressively so that the
group\'s kinetic energy would go to zero, or any individual atom\'s
kinetic energy would go to zero for the case where *eflux* is an
atom-style variable, then LAMMPS will halt with an error message.
::::

Fix heat is different from a thermostat such as [fix nvt](fix_nh) or
[fix temp/rescale](fix_temp_rescale) in that energy is added/subtracted
continually. Thus if there is not 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)) 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_nh) or [fix langevin](fix_langevin) fix.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix.

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). This scalar is the most recent value by
which velocities were scaled. The scalar value calculated by this fix is
\"intensive\". If *eflux* is specified as an atom-style variable, this
fix computes the average value by which the velocities were scaled for
all of the atoms that had their velocities scaled.

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

> none

## Related commands

[fix ehex](fix_ehex), [compute temp](compute_temp), [compute
temp/region](compute_temp_region)

## Default

none