lammps/doc/fix_nvt.txt

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fix nvt command :h3

[Syntax:]

fix ID group-ID nvt Tstart Tstop Tdamp keyword value ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
nvt = style name of this fix command :l
Tstart,Tstop = desired temperature at start/end of run :l
Tdamp = temperature damping parameter (time units) :l

zero or more keyword/value pairs may be appended to the args :l
keyword = {drag} :l
  {drag} value = drag factor added to thermostat (0.0 = no drag) :pre
:ule

[Examples:]

fix 1 all nvt 300.0 300.0 100.0
fix 1 all nvt 300.0 300.0 100.0 drag 0.2 :pre

[Description:]

Perform constant NVT integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover temperature
thermostat.  V is volume; T is temperature.  This creates a system
trajectory consistent with the canonical ensemble.

The desired temperature at each timestep is a ramped value during the
run from {Tstart} to {Tstop}.  The "run"_run.html command documents
how to make the ramping take place across multiple runs.  The {Tdamp}
parameter is specified in time units and determines how rapidly the
temperature is relaxed.  For example, a value of 100.0 means to relax
the temperature in a timespan of (roughly) 100 time units (tau or
fmsec or psec - see the "units"_units.html command).

In some cases (e.g. for solids) the temperature of the system can
oscillate undesirably when a Nose/Hoover thermostat is applied.  The
optional {drag} keyword will damp these oscillations, although it
alters the Nose/Hoover equations.  A value of 0.0 (no drag) leaves the
Nose/Hoover formalism unchanged.  A non-zero value adds a drag term;
the larger the value specified, the greater the damping effect.
Performing a short run and monitoring the temperature is the best way
to determine if the drag term is working.  Typically a value between
0.2 to 2.0 is sufficient to damp oscillations after a few periods.

This fix computes a temperature each timestep.  To do this, the fix
creates its own compute of style "temp", as if this command had been
issued:

compute fix-ID_temp group-ID temp :pre

See the "compute temp"_compute_temp.html command for details.  Note
that the ID of the new compute is the fix-ID with underscore + "temp"
appended and the group for the new compute is the same as the fix
group.

Note that this is NOT the compute used by thermodynamic output (see
the "thermo_style"_thermo_style.html command) with ID = {thermo_temp}.
This means you can change the attributes of this fix's temperature
(e.g. its degrees-of-freedom) via the
"compute_modify"_compute_modify.html command or print this temperature
during thermodyanmic output via the "thermo_style
custom"_thermo_style.html command using the appropriate compute-ID.
It also means that changing attributes of {thermo_temp} will have no
effect on this fix.  Alternatively, you can directly assign a new
compute (for calculating temeperature) that you have defined to this
fix via the "fix_modify"_fix_modify.html command.

This fix makes a contribution to the potential energy of the system
that can be included in thermodynamic output of potential energy using
the "fix_modify energy"_fix_modify.html option.  The contribution can
also be printed by itself via the keyword {f_fix-ID} in the
"thermo_style custom"_thermo_style.html command.

[Restrictions:]

The final Tstop cannot be 0.0 since it would make the target T = 0.0
at some timestep during the simulation which is not allowed in 
the Nose/Hoover formulation.

[Related commands:]

"fix nve"_fix_nve.html, "fix npt"_fix_npt.html, "fix
temp/rescale"_fix_temp_rescale.html, "fix langevin"_fix_langevin.html,
"fix_modify"_fix_modify.html, "temperature"_temperature.html

[Default:]

The keyword defaults are drag = 0.0.