.. index:: compute temp
.. index:: compute temp/kk

compute temp command
====================

Accelerator Variants: *temp/kk*

Syntax
""""""

.. code-block:: LAMMPS

   compute ID group-ID temp

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

Examples
""""""""

.. code-block:: LAMMPS

   compute 1 all temp
   compute myTemp mobile temp

Description
"""""""""""

Define a computation that calculates the temperature of a group of
atoms.  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>`,
etc.

The temperature is calculated by the formula

.. math::

   T  = \frac{2 E_\mathrm{kin}}{N_\mathrm{DOF} k_B} \quad \mathrm{with} \quad
   E_\mathrm{kin} = \sum^{N_\mathrm{atoms}}_{i=1} \frac{1}{2} m_i v^2_i \quad \mathrm{and} \quad
   N_\mathrm{DOF} = n_\mathrm{dim} N_\mathrm{atoms} - n_\mathrm{dim} - N_\mathrm{fix DOFs}

where :math:`E_\mathrm{kin}` is the total kinetic energy of the group of
atoms, :math:`n_\mathrm{dim}` is the dimensionality of the simulation
(i.e. either 2 or 3), :math:`N_\mathrm{atoms}` is the number of atoms in
the group, :math:`N_\mathrm{fix DOFs}` is the number of degrees of
freedom removed by fix commands (see below), :math:`k_B` is the
Boltzmann constant, and :math:`T` is the resulting computed 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.

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.
By default this *extra* component is initialized to
:math:`n_\mathrm{dim}` (as shown in the formula above) to represent the
degrees of freedom removed from a system due to its translation
invariance due to periodic boundary conditions.

A compute of this style with the ID of "thermo_temp" is created when
LAMMPS starts up, as if this command were in the input script:

.. code-block:: LAMMPS

   compute thermo_temp all temp

See the "thermo_style" command for more details.

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

----------

.. include:: accel_styles.rst

----------

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

This compute calculates a global scalar (the temperature) and a global
vector of length six (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 are in energy :doc:`units <units>`.

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

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

:doc:`compute temp/partial <compute_temp_partial>`,
:doc:`compute temp/region <compute_temp_region>`,
:doc:`compute pressure <compute_pressure>`

Default
"""""""

none
