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rLAMMPS lammps
compute_temp_com.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>
compute temp/com command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
compute ID group-ID temp/com
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"compute.html"
>
compute
</A>
command
<LI>
temp/com = style name of this compute command
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
compute 1 all temp/com
compute myTemp mobile temp/com
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Define a computation that calculates the temperature of a group of
atoms, after subtracting out the center-of-mass velocity of the group.
This is useful if the group is expected to have a non-zero net
velocity for some reason. A compute of this style can be used by any
command that computes a temperature,
e.g.
<A
HREF =
"thermo_modify.html"
>
thermo_modify
</A>
,
<A
HREF =
"fix_temp_rescale.html"
>
fix
temp/rescale
</A>
,
<A
HREF =
"fix_nh.html"
>
fix npt
</A>
, etc.
</P>
<P>
After the center-of-mass velocity has been subtracted from each atom,
the temperature is calculated by the formula KE = dim/2 N k T, where
KE = total kinetic energy of the group of atoms (sum of 1/2 m v^2),
dim = 2 or 3 = dimensionality of the simulation, N = number of atoms
in the group, k = Boltzmann constant, and T = temperature.
</P>
<P>
A kinetic energy tensor, stored as a 6-element vector, is also
calculated by this compute for use in the computation of a pressure
tensor. The formula for the components of the tensor is the same as
the above formula, except that v^2 is replaced by vx*vy for the xy
component, etc. The 6 components of the vector are ordered xx, yy,
zz, xy, xz, yz.
</P>
<P>
The number of atoms contributing to the temperature is assumed to be
constant for the duration of the run; use the
<I>
dynamic
</I>
option of the
<A
HREF =
"compute_modify.html"
>
compute_modify
</A>
command if this is not the case.
</P>
<P>
The removal of the center-of-mass velocity by this fix is essentially
computing the temperature after a "bias" has been removed from the
velocity of the atoms. If this compute is used with a fix command
that performs thermostatting then this bias will be subtracted from
each atom, thermostatting of the remaining thermal velocity will be
performed, and the bias will be added back in. Thermostatting fixes
that work in this way include
<A
HREF =
"fix_nh.html"
>
fix nvt
</A>
,
<A
HREF =
"fix_temp_rescale.html"
>
fix
temp/rescale
</A>
,
<A
HREF =
"fix_temp_berendsen.html"
>
fix
temp/berendsen
</A>
, and
<A
HREF =
"fix_langevin.html"
>
fix
langevin
</A>
.
</P>
<P>
This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as
<A
HREF =
"fix_shake.html"
>
fix shake
</A>
and
<A
HREF =
"fix_rigid.html"
>
fix rigid
</A>
. This means the temperature of groups of
atoms that include these constraints will be computed correctly. If
needed, the subtracted degrees-of-freedom can be altered using the
<I>
extra
</I>
option of the
<A
HREF =
"compute_modify.html"
>
compute_modify
</A>
command.
</P>
<P>
See
<A
HREF =
"Section_howto.html#howto_16"
>
this howto section
</A>
of the manual for
a discussion of different ways to compute temperature and perform
thermostatting.
</P>
<P><B>
Output info:
</B>
</P>
<P>
This compute calculates a global scalar (the temperature) and a global
vector of length 6 (KE tensor), which can be accessed by indices 1-6.
These values can be used by any command that uses global scalar or
vector values from a compute as input. See
<A
HREF =
"Section_howto.html#howto_15"
>
this
section
</A>
for an overview of LAMMPS output
options.
</P>
<P>
The scalar value calculated by this compute is "intensive". The
vector values are "extensive".
</P>
<P>
The scalar value will be in temperature
<A
HREF =
"units.html"
>
units
</A>
. The
vector values will be in energy
<A
HREF =
"units.html"
>
units
</A>
.
</P>
<P><B>
Restrictions:
</B>
none
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"compute_temp.html"
>
compute temp
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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