forked from lijiext/lammps
92 lines
3.6 KiB
Plaintext
92 lines
3.6 KiB
Plaintext
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"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 to the args :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. V is volume; T is temperature. This creates a system
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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 "run"_run.html command documents
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how to make the ramping take place across multiple runs. The {Tdamp}
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parameter is specified in time units and determines how rapidly the
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temperature is relaxed. For example, a value of 100.0 means to relax
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the temperature in a timespan of (roughly) 100 time units (tau or
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fmsec or psec - see the "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. The fix creates its
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own method for computing T, as if it had been defined by the command:
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temperature fix-ID group-ID full :pre
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See the "temperature"_temperature.html command for details. Note that
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this is NOT the temperature with ID = {default}. This means you can
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change the attributes of this fix's temperature (e.g. its
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degrees-of-freedom) via the "temp_modify"_temp_modify.html command or
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print the temperature with thermodyanmic output via the "thermo_style
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custom"_thermo_style.html command using the appropriate temp-ID =
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fix-ID. It also means that changing attributes of the default
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temperature will have no effect on this fix. Alternatively, you can
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directly assign a new temperature to the fix via the
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"fix_modify"_fix_modify.html command.
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This fix supports the "fix_modify"_fix_modify.html options for
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{thermo} and {energy}. The former will print the contribution the fix
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makes to the energy of the system when thermodynamics is printed. The
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latter will add this contribution to the total potential energy
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(PotEng) so that energy conservation can be monitored.
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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, "temperature"_temperature.html
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[Default:]
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The keyword defaults are drag = 0.0.
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