lammps/doc/compute_temp_com.txt

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compute temp/com command :h3

[Syntax:]

compute ID group-ID temp/com :pre

ID, group-ID are documented in "compute"_compute.html command
temp/com = style name of this compute command :ul

[Examples:]

compute 1 all temp/com
compute myTemp mobile temp/com :pre

[Description:]

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. "thermo_modify"_thermo_modify.html, "fix
temp/rescale"_fix_temp_rescale.html, "fix npt"_fix_npt.html, etc.

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.

A 6-component kinetic energy tensor 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 number of atoms contributing to the temperature is assumed to be
constant for the duration of the run; use the {dynamic} option of the
"compute_modify"_compute_modify.html command if this is not the case.

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 "fix nvt"_fix_nvt.html, "fix
temp/rescale"_fix_temp_rescale.html, "fix
temp/berendsen"_fix_temp_berendsen, and "fix
langevin"_fix_langevin.html.

This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as "fix shake"_fix_shake.html and
"fix rigid"_fix_rigid.html.  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
{extra} option of the "compute_modify"_compute_modify.html command.

See "this howto section"_Section_howto.html#4_16 of the manual for a
discussion of different ways to compute temperature and perform
thermostatting.

[Output info:]

The scalar value calculated by this compute is "intensive", meaning it
is independent of the number of atoms in the simulation.  The vector
values are "extensive", meaning they scale with the number of atoms in
the simulation.

[Restrictions:] none

[Related commands:]

"compute temp"_compute_temp.html

[Default:] none