forked from lijiext/lammps
172 lines
7.2 KiB
Plaintext
172 lines
7.2 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 temp/rescale command :h3
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fix temp/rescale/cuda command :h3
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fix temp/rescale/limit/cuda command :h3
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[Syntax:]
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fix ID group-ID temp/rescale N Tstart Tstop window fraction :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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temp/rescale = style name of this fix command :l
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N = perform rescaling every N steps :l
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Tstart,Tstop = desired temperature at start/end of run (temperature units) :l
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Tstart can be a variable (see below) :pre
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window = only rescale if temperature is outside this window (temperature units) :l
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fraction = rescale to target temperature by this fraction :l,ule
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[Examples:]
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fix 3 flow temp/rescale 100 1.0 1.1 0.02 0.5
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fix 3 boundary temp/rescale 1 1.0 1.5 0.05 1.0
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fix 3 boundary temp/rescale 1 1.0 1.5 0.05 1.0 :pre
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[Description:]
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Reset the temperature of a group of atoms by explicitly rescaling
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their velocities.
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The rescaling is applied to only the translational degrees of freedom
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for the particles, which is an important consideration if finite-size
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particles which have rotational degrees of freedom are being
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thermostatted with this fix. The translational degrees of freedom can
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also have a bias velocity removed from them before thermostatting
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takes place; see the description below.
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Rescaling is performed every N timesteps. The target temperature is a
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ramped value between the {Tstart} and {Tstop} temperatures at the
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beginning and end of the run.
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{Tstart} can be specified as an equal-style "variable"_variable.html.
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In this case, the {Tstop} setting is ignored. 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 used to determine the target temperature.
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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 temperature.
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Rescaling is only performed if the difference between the current and
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desired temperatures is greater than the {window} value. The amount
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of rescaling that is applied is a {fraction} (from 0.0 to 1.0) of the
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difference between the actual and desired temperature. E.g. if
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{fraction} = 1.0, the temperature is reset to exactly the desired
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value.
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IMPORTANT NOTE: Unlike the "fix nvt"_fix_nh.html command which
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performs Nose/Hoover thermostatting AND time integration, this fix
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does NOT perform time integration. It only modifies velocities to
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effect thermostatting. Thus you must use a separate time integration
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fix, like "fix nve"_fix_nve.html to actually update the positions of
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atoms using the modified velocities. Likewise, this fix should not
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normally be used on atoms that also have their temperature controlled
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by another fix - e.g. by "fix nvt"_fix_nh.html or "fix
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langevin"_fix_langevin.html commands.
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See "this howto section"_Section_howto.html#howto_16 of the manual for
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a discussion of different ways to compute temperature and perform
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thermostatting.
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This fix computes a temperature each timestep. To do this, the fix
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creates its own compute of style "temp", as if one of this command had
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been issued:
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compute fix-ID_temp group-ID temp :pre
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See the "compute temp"_compute_temp.html for details. Note that the
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ID of the new compute is the fix-ID + underscore + "temp", and the
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group for the new compute is the same as the fix group.
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Note that this is NOT the compute used by thermodynamic output (see
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the "thermo_style"_thermo_style.html command) with ID = {thermo_temp}.
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This means you can change the attributes of this fix's temperature
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(e.g. its degrees-of-freedom) via the
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"compute_modify"_compute_modify.html command or print this temperature
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during thermodynamic output via the "thermo_style
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custom"_thermo_style.html command using the appropriate compute-ID.
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It also means that changing attributes of {thermo_temp} will have no
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effect on this fix.
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Like other fixes that perform thermostatting, this fix can be used
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with "compute commands"_compute.html that calculate a temperature
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after removing a "bias" from the atom velocities. E.g. removing the
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center-of-mass velocity from a group of atoms or only calculating
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temperature on the x-component of velocity or only calculating
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temperature for atoms in a geometric region. This is not done by
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default, but only if the "fix_modify"_fix_modify.html command is used
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to assign a temperature compute to this fix that includes such a bias
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term. See the doc pages for individual "compute
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commands"_compute.html to determine which ones include a bias. In
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this case, the thermostat works in the following manner: the current
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temperature is calculated taking the bias into account, bias is
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removed from each atom, thermostatting is performed on the remaining
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thermal degrees of freedom, and the bias is added back in.
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:line
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Styles with a {cuda} suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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"Section_accelerate"_Section_accelerate.html of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.
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These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the "-suffix command-line
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switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
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use the "suffix"_suffix.html command in your input script.
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See "Section_accelerate"_Section_accelerate.html of the manual for
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more instructions on how to use the accelerated styles effectively.
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:line
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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.
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The "fix_modify"_fix_modify.html {temp} option is supported by this
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fix. You can use it to assign a temperature "compute"_compute.html
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you have defined to this fix which will be used in its thermostatting
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procedure, as described above. For consistency, the group used by
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this fix and by the compute should be the same.
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The "fix_modify"_fix_modify.html {energy} option is supported by this
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fix to add the energy change implied by a velocity rescaling to the
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system's potential energy as part of "thermodynamic
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output"_thermo_style.html.
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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. The scalar is the
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cummulative energy change due to this fix. The scalar value
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calculated by this fix is "extensive".
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This fix can ramp its target temperature over multiple runs, using the
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{start} and {stop} keywords of the "run"_run.html command. See the
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"run"_run.html command for details of how to do this.
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This fix is not invoked during "energy minimization"_minimize.html.
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[Restrictions:] none
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[Related commands:]
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"fix langevin"_fix_langevin.html, "fix nvt"_fix_nh.html,
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"fix_modify"_fix_modify.html
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[Default:] none
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