forked from lijiext/lammps
130 lines
5.2 KiB
Plaintext
130 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 temp/berendsen command :h3
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[Syntax:]
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fix ID group-ID temp/berendsen Tstart Tstop Tdamp :pre
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ID, group-ID are documented in "fix"_fix.html command
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temp/berendsen = style name of this fix command
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Tstart,Tstop = desired temperature at start/end of run
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Tdamp = temperature damping parameter (time units) :ul
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[Examples:]
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fix 1 all nvt 300.0 300.0 100.0 :pre
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[Description:]
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Reset the temperature of a group of atoms by using a Berendsen
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thermostat "(Berendsen)"_#Berendsen, which rescales their velocities
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every timestep.
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The thermostat is applied to only the translational degrees of freedom
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for the particles, which is an important consideration if extended
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spherical or aspherical particles which have rotational degrees of
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freedom are being thermostatted with this fix. The translational
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degrees of freedom can also have a bias velocity removed from them
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before thermostatting takes place; see the description below.
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The desired temperature at each timestep is a ramped value during the
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run from {Tstart} to {Tstop}. The {Tdamp} parameter is specified in
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time units and determines how rapidly the temperature is relaxed. For
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example, a value of 100.0 means to relax the temperature in a timespan
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of (roughly) 100 time units (tau or fmsec or psec - see the
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"units"_units.html command).
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IMPORTANT NOTE: Unlike the "fix nvt"_fix_nvt.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_nvt.html or "fix
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langevin"_fix_langevin.html commands.
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See "this howto section"_Section_howto.html#4_16 of the manual for a
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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 this command had been
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issued:
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compute fix-ID_temp group-ID temp :pre
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See the "compute temp"_compute_temp.html command for details. Note
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that the ID of the new compute is the fix-ID + underscore + "temp",
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and the 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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[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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No global scalar or vector or per-atom quantities are stored by this
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fix for access by various "output commands"_Section_howto.html#4_15.
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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 nve"_fix_nve.html, "fix nvt"_fix_nvt.html, "fix
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temp/rescale"_fix_temp_rescale.html, "fix langevin"_fix_langevin.html,
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"fix_modify"_fix_modify.html, "compute temp"_compute_temp.html,
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"fix press/berendsen"_fix_press_berendsen.html
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[Default:] none
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:line
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:link(Berendsen)
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[(Berendsen)] Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem
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Phys, 81, 3684 (1984).
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