forked from lijiext/lammps
124 lines
4.6 KiB
Plaintext
124 lines
4.6 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 nvt command :h3
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[Syntax:]
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fix ID group-ID nvt Tstart Tstop Tdamp keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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nvt = style name of this fix command :l
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Tstart,Tstop = desired temperature at start/end of run :l
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Tdamp = temperature damping parameter (time units) :l
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zero or more keyword/value pairs may be appended :l
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keyword = {drag} :l
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{drag} value = drag factor added to thermostat (0.0 = no drag) :pre
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:ule
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[Examples:]
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fix 1 all nvt 300.0 300.0 100.0
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fix 1 all nvt 300.0 300.0 100.0 drag 0.2 :pre
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[Description:]
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Perform constant NVT integration to update positions and velocities
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each timestep for atoms in the group using a Nose/Hoover temperature
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thermostat "(Hoover)"_#Hoover. V is volume; T is temperature. This
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creates a system trajectory consistent with the canonical ensemble.
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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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In some cases (e.g. for solids) the temperature of the system can
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oscillate undesirably when a Nose/Hoover thermostat is applied. The
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optional {drag} keyword will damp these oscillations, although it
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alters the Nose/Hoover equations. A value of 0.0 (no drag) leaves the
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Nose/Hoover formalism unchanged. A non-zero value adds a drag term;
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the larger the value specified, the greater the damping effect.
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Performing a short run and monitoring the temperature is the best way
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to determine if the drag term is working. Typically a value between
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0.2 to 2.0 is sufficient to damp oscillations after a few periods.
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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 thermodyanmic 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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[Restart, fix_modify, output, run start/stop, minimize info:]
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This fix writes the state of the Nose/Hoover thermostat to "binary
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restart files"_restart.html. See the "read_restart"_read_restart.html
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command for info on how to re-specify a fix in an input script that
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reads a restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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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 "compute"_compute.html you have
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defined to this fix which will be used in its thermostatting
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procedure.
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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 induced by Nose/Hoover thermostatting to
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the system's potential energy as part of "thermodynamic
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output"_thermo_style.html.
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The potential energy change due to this fix is stored as a scalar
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quantity, which can be accessed by various "output
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commands"_Section_howto.html#4_15. The scalar value calculated by
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this fix is "extensive", meaning it scales with the number of atoms in
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the simulation.
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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:]
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The final Tstop cannot be 0.0 since it would make the target T = 0.0
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at some timestep during the simulation which is not allowed in
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the Nose/Hoover formulation.
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[Related commands:]
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"fix nve"_fix_nve.html, "fix npt"_fix_npt.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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[Default:]
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The keyword defaults are drag = 0.0.
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:line
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:link(Hoover)
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[(Hoover)] Hoover, Phys Rev A, 31, 1695 (1985).
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