fix heat command


fix ID group-ID heat N eflux
  • ID, group-ID are documented in 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


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


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 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 a variable (see below). If it is a numeric constant or equal-style variable which evaluates to a scalar value, then the eflux determines the change in aggregate energy of the entire group of atoms per unit time, e.g. in eV/psec for metal 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 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.


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 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.

As mentioned above, the eflux parameter can be specified as an equal-style or atom_style 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 value(s) used to determine the flux.

Equal-style variables can specify formulas with various mathematical functions, and include 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.


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 or fix temp/rescale 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 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) 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 or fix langevin fix.

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

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

This fix computes a global scalar which can be accessed by various output commands. This scalar is the most recent value by which velocites 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 command. This fix is not invoked during energy minimization.