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 :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 "(Hoover)"_#Hoover.  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 {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 + underscore + "temp",
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.

[Restart, fix_modify, output, run start/stop, minimize info:]

This fix writes the state of the Nose/Hoover thermostat to "binary
restart files"_restart.html.  See the "read_restart"_read_restart.html
command for info on how to re-specify a fix in an input script that
reads a restart file, so that the operation of the fix continues in an
uninterrupted fashion.

The "fix_modify"_fix_modify.html {temp} option is supported by this
fix.  You can use it to assign a "compute"_compute.html you have
defined to this fix which will be used in its thermostatting
procedure.

The "fix_modify"_fix_modify.html {energy} option is supported by this
fix to add the energy change induced by Nose/Hoover thermostatting to
the system's potential energy as part of "thermodynamic
output"_thermo_style.html.

The potential energy change due to this fix is stored as a scalar
quantity, which can be accessed by various "output
commands"_Section_howto.html#4_15.  The scalar value calculated by
this fix is "extensive", meaning it scales with the number of atoms in
the simulation.

This fix can ramp its target temperature over multiple runs, using the
{start} and {stop} keywords of the "run"_run.html command.  See the
"run"_run.html command for details of how to do this.

This fix is not invoked during "energy minimization"_minimize.html.

[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, "compute temp"_compute_temp.html

[Default:]

The keyword defaults are drag = 0.0.

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:link(Hoover)
[(Hoover)] Hoover, Phys Rev A, 31, 1695 (1985).