"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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fix nvt/asphere command :h3

[Syntax:]

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

ID, group-ID are documented in "fix"_fix.html command :ulb,l
nvt/asphere = 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/asphere 300.0 300.0 100.0
fix 1 all nvt/asphere 300.0 300.0 100.0 drag 0.2 :pre

[Description:]

Perform constant NVT integration to update positions, velocities,
and angular velocities each timestep for aspherical or ellipsoidal 
particles 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 {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/asphere", as if this command 
had been issued:

compute fix-ID_temp group-ID temp/asphere :pre

See the "compute temp/asphere"_compute_temp_asphere.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.

[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.

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:]

This fix is part of the "asphere" package.  It is only enabled if
LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_3 section for more info.

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 nvt"_fix_nvt.html, "fix nve_asphere"_fix_nve_asphere.html, "fix
npt_asphere"_fix_npt_asphere.html, "fix_modify"_fix_modify.html

[Default:]

The keyword defaults are drag = 0.0.
