forked from lijiext/lammps
125 lines
5.2 KiB
Plaintext
125 lines
5.2 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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eflux can be a variable (see below) :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 3 qin heat 10 v_flux
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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 in a
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manner that conserves their aggregate momentum. Two of these fixes
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can be used to establish a temperature gradient across a simulation
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domain by adding heat (energy) to one group of atoms (hot reservoir)
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and 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 can be specified as a numeric constant or as a variable (see
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below). If it is a numeric constant or equal-style variable which
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evaluates to a scalar value, then the {eflux} determines the change in
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aggregate energy of the entire group of atoms per unit time, e.g. in
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eV/psec for "metal units"_units.html. In this case it is an
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"extensive" quantity, meaning its magnitude should be scaled with the
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number of atoms in the group. Note that since {eflux} has per-time
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units (i.e. it is a flux), this means that a larger value of N will
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add/subtract a larger amount of energy each time the fix is invoked.
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If {eflux} is specified as an atom-style variable (see below), then
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the variable computes one value per atom. In this case, each value is
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the energy flux for a single atom, again in units of energy per unit
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time. In this case, each value is an "intensive" quantity, which need
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not be scaled with the number of atoms in the group.
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As mentioned above, the {eflux} parameter can be specified as an
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equal-style or atom_style "variable"_variable.html. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value(s) used to determine the flux.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent flux.
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Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent flux
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with optional time-dependence as well.
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IMPORTANT NOTE: If heat is subtracted from the system too aggressively
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so that the group's kinetic energy would go to zero, or any individual
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atom's kinetic energy would go to zero for the case where {eflux} is
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an atom-style variable, then LAMMPS will halt 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#howto_15. This scalar is the
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most recent value by which velocites were scaled. The scalar value
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calculated by this fix is "intensive". If {eflux} is specified as
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an atom-style variable, this fix computes the average value by which
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the velocities were scaled for all of the atoms that had their
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velocities scaled.
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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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