forked from lijiext/lammps
86 lines
3.1 KiB
Plaintext
86 lines
3.1 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix heat command :h3
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[Syntax:]
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fix ID group-ID heat N eflux :pre
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ID, group-ID are documented in "fix"_fix.html command
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heat = style name of this fix command
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N = add/subtract heat every this many timesteps
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eflux = rate of heat addition or subtraction (energy/time units) :ul
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[Examples:]
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fix 3 qin heat 1 1.0
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fix 4 qout heat 1 -1.0 :pre
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[Description:]
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Add non-translational kinetic energy (heat) to a group of atoms such
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that their aggregate momentum is conserved. Two of these fixes can be
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used to establish a temperature gradient across a simulation domain by
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adding heat to one group of atoms (hot reservoir) and subtracting heat
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from another (cold reservoir). E.g. a simulation sampling from the
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McDLT ensemble. Note that the fix is applied to a group of atoms, not
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a geometric region, so that the same set of atoms is affected wherever
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they may move to.
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Heat addition/subtraction is performed every N timesteps. The {eflux}
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parameter determines the change in aggregate energy of the entire
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group of atoms. Thus it is an "extensive" quantity, meaning its
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magnitude should be scaled with the number of atoms in the group.
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Since eflux is in units of energy/time, this means a larger value of N
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will add/subtract a larger amount of energy each timestep the fix is
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invoked. If heat is subtracted from the system too aggressively so
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that the group's kinetic energy goes to zero, LAMMPS halts with an
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error message.
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Fix heat is different from a thermostat such as "fix nvt"_fix_nvt.html
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or "fix temp/rescale"_fix_temp_rescale.html in that energy is
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added/subtracted continually. Thus if there isn't another mechanism
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in place to counterbalance this effect, the entire system will heat or
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cool continuously. You can use multiple heat fixes so that the net
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energy change is 0.0 or use "fix viscous"_fix_viscous to drain energy
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from the system.
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This fix does not change the coordinates of its atoms; it only scales
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their velocities. Thus you must still use an integration fix
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(e.g. "fix nve"_fix_nve.html) on the affected atoms. This fix should
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not normally be used on atoms that have their temperature controlled
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by another fix - e.g. "fix nvt"_fix_nvt.html or "fix
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langevin"_fix_langevin.html fix.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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This fix computes a scalar which can be accessed by various "output
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commands"_Section_howto.html#4_15. This scalar is the most recent
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value by which velocites were scaled. The scalar value calculated by
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this fix is "intensive", meaning it is independent of the number of
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atoms in the simulation.
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:] none
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[Related commands:]
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"compute temp"_compute_temp.html, "compute
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temp/region"_compute_temp_region.html
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[Default:] none
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