.. index:: compute temp/com

compute temp/com command
========================

Syntax
""""""

.. code-block:: LAMMPS

   compute ID group-ID temp/com

* ID, group-ID are documented in :doc:`compute <compute>` command
* temp/com = style name of this compute command

Examples
""""""""

.. code-block:: LAMMPS

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

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., :doc:`thermo_modify <thermo_modify>`,
:doc:`fix temp/rescale <fix_temp_rescale>`, :doc:`fix npt <fix_nh>`).

After the center-of-mass velocity has been subtracted from each atom,
the temperature is calculated by the formula

.. math::

   \text{KE} = \frac{\text{dim}}{2} N k_B T,

where KE is the total kinetic energy of the group of atoms (sum of
:math:`\frac12 m v^2`), dim = 2 or 3 is the dimensionality of the
simulation, :math:`N` is number of atoms in the group, :math:`k_B` is
the Boltzmann constant, and :math:`T` is the absolute temperature.

A symmetric tensor, stored as a six-element vector, is also calculated
by this compute for use in the computation of a pressure tensor by the
:doc:`compute pressue <compute_pressure>` command.  The formula for
the components of the tensor is the same as the above expression for
:math:`E_\mathrm{kin}`, except that the 1/2 factor is NOT included and
the :math:`v_i^2` is replaced by :math:`v_{i,x} v_{i,y}` for the
:math:`xy` component, and so on.  Note that because it lacks the 1/2
factor, these tensor components are twice those of the traditional
kinetic energy tensor.  The six components of the vector are ordered
:math:`xx`, :math:`yy`, :math:`zz`, :math:`xy`, :math:`xz`,
:math:`yz`.

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
:doc:`compute_modify <compute_modify>` 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 :doc:`fix nvt <fix_nh>`,
:doc:`fix temp/rescale <fix_temp_rescale>`,
:doc:`fix temp/berendsen <fix_temp_berendsen>`, and
:doc:`fix langevin <fix_langevin>`.

This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as :doc:`fix shake <fix_shake>` and
:doc:`fix rigid <fix_rigid>`.  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 :doc:`compute_modify <compute_modify>` command.

See the :doc:`Howto thermostat <Howto_thermostat>` page for a
discussion of different ways to compute temperature and perform
thermostatting.

Output info
"""""""""""

This compute calculates a global scalar (the temperature) and a global
vector of length 6 (symmetric 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 the
:doc:`Howto output <Howto_output>` page for an overview of LAMMPS
output options.

The scalar value calculated by this compute is "intensive".  The
vector values are "extensive".

The scalar value is in temperature :doc:`units <units>`.  The vector
values is in energy :doc:`units <units>`.

Restrictions
""""""""""""
 none

Related commands
""""""""""""""""

:doc:`compute temp <compute_temp>`

Default
"""""""

none
