forked from lijiext/lammps
94 lines
3.5 KiB
Plaintext
94 lines
3.5 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 :ulb,l
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heat = style name of this fix command :l
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N = add/subtract heat every this many timesteps :l
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eflux = rate of heat addition or subtraction (energy/time units) :l
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zero or more keyword/value pairs may be appended to args :l
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keyword = {region} :l
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{region} value = region-ID
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region-ID = ID of region atoms must be in to have added force :pre
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:ule
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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 region top :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 (energy) to one group of atoms (hot reservoir) and
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subtracting heat from another (cold reservoir). E.g. a simulation
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sampling from the McDLT ensemble.
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If the {region} keyword is used, the atom must be in both the group
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and the specified geometric "region"_region.html in order to have
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energy added or subtracted to it. If not specified, then the atoms in
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the group are affected wherever 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 per unit time, e.g. in eV/psec for "metal
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units"_units.html. 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 independent of N or the "timestep"_timestep.html, a
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larger value of N will add/subtract a larger amount of energy each
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time the fix is invoked. If heat is subtracted from the system too
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aggressively so that the group's kinetic energy would go to zero,
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LAMMPS halts with an error message.
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Fix heat is different from a thermostat such as "fix nvt"_fix_nh.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.html to drain
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energy 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_nh.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 global scalar which can be accessed by various
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"output commands"_Section_howto.html#4_15. This scalar is the most
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recent value by which velocites were scaled. The scalar value
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calculated by this fix is "intensive".
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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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