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rLAMMPS lammps
fix_nvt.html
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<HTML>
<CENTER><A
HREF =
"http://lammps.sandia.gov"
>
LAMMPS WWW Site
</A>
-
<A
HREF =
"Manual.html"
>
LAMMPS Documentation
</A>
-
<A
HREF =
"Section_commands.html#comm"
>
LAMMPS Commands
</A>
</CENTER>
<HR>
<H3>
fix nvt command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
fix ID group-ID nvt Tstart Tstop Tdamp keyword value ...
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"fix.html"
>
fix
</A>
command
<LI>
nvt = style name of this fix command
<LI>
Tstart,Tstop = desired temperature at start/end of run
<LI>
Tdamp = temperature damping parameter (time units)
<LI>
zero or more keyword/value pairs may be appended
<LI>
keyword =
<I>
drag
</I>
<PRE>
<I>
drag
</I>
value = drag factor added to thermostat (0.0 = no drag)
</PRE>
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
fix 1 all nvt 300.0 300.0 100.0
fix 1 all nvt 300.0 300.0 100.0 drag 0.2
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Perform constant NVT integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover temperature
thermostat
<A
HREF =
"#Hoover"
>
(Hoover)
</A>
. V is volume; T is temperature. This
creates a system trajectory consistent with the canonical ensemble.
</P>
<P>
The desired temperature at each timestep is a ramped value during the
run from
<I>
Tstart
</I>
to
<I>
Tstop
</I>
. The
<I>
Tdamp
</I>
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
<A
HREF =
"units.html"
>
units
</A>
command).
</P>
<P>
In some cases (e.g. for solids) the temperature of the system can
oscillate undesirably when a Nose/Hoover thermostat is applied. The
optional
<I>
drag
</I>
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.
</P>
<P>
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:
</P>
<PRE>
compute fix-ID_temp group-ID temp
</PRE>
<P>
See the
<A
HREF =
"compute_temp.html<A HREF = "
temp
""
>
>compute temp
</A>
command for details. Note
that the ID of the new compute is the fix-ID with
</A>
appended and
the group for the new compute is the same as the fix group.
</P>
<P>
Note that this is NOT the compute used by thermodynamic output (see
the
<A
HREF =
"thermo_style.html"
>
thermo_style
</A>
command) with ID =
<I>
thermo_temp
</I>
.
This means you can change the attributes of this fix's temperature
(e.g. its degrees-of-freedom) via the
<A
HREF =
"compute_modify.html"
>
compute_modify
</A>
command or print this temperature
during thermodyanmic output via the
<A
HREF =
"thermo_style.html"
>
thermo_style
custom
</A>
command using the appropriate compute-ID.
It also means that changing attributes of
<I>
thermo_temp
</I>
will have no
effect on this fix.
</P>
<P><B>
Restart, fix_modify, output, run start/stop, minimize info:
</B>
</P>
<P>
This fix writes the state of the Nose/Hoover thermostat to
<A
HREF =
"restart.html"
>
binary
restart files
</A>
. See the
<A
HREF =
"read_restart.html"
>
read_restart
</A>
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.
</P>
<P>
The
<A
HREF =
"fix_modify.html"
>
fix_modify
</A>
<I>
temp
</I>
option is supported by this
fix. You can use it to assign a
<A
HREF =
"compute.html"
>
compute
</A>
you have
defined to this fix which will be used in its thermostatting
procedure.
</P>
<P>
The
<A
HREF =
"fix_modify.html"
>
fix_modify
</A>
<I>
energy
</I>
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
<A
HREF =
"thermo_style.html"
>
thermodynamic
output
</A>
.
</P>
<P>
The potential energy change due to this fix is stored as a scalar
quantity, which can be accessed by various
<A
HREF =
"Section_howto.html#4_15"
>
output
commands
</A>
. The scalar value calculated by
this fix is "extensive", meaning it scales with the number of atoms in
the simulation.
</P>
<P>
This fix can ramp its target temperature over multiple runs, using the
<I>
start
</I>
and
<I>
stop
</I>
keywords of the
<A
HREF =
"run.html"
>
run
</A>
command. See the
<A
HREF =
"run.html"
>
run
</A>
command for details of how to do this.
</P>
<P>
This fix is not invoked during
<A
HREF =
"minimize.html"
>
energy minimization
</A>
.
</P>
<P><B>
Restrictions:
</B>
</P>
<P>
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.
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"fix_nve.html"
>
fix nve
</A>
,
<A
HREF =
"fix_npt.html"
>
fix npt
</A>
,
<A
HREF =
"fix_temp_rescale.html"
>
fix
temp/rescale
</A>
,
<A
HREF =
"fix_langevin.html"
>
fix langevin
</A>
,
<A
HREF =
"fix_modify.html"
>
fix_modify
</A>
,
<A
HREF =
"temperature.html"
>
temperature
</A>
</P>
<P><B>
Default:
</B>
</P>
<P>
The keyword defaults are drag = 0.0.
</P>
<HR>
<A
NAME =
"Hoover"
></A>
<P><B>
(Hoover)
</B>
Hoover, Phys Rev A, 31, 1695 (1985).
</P>
</HTML>
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