.. index:: fix temp/berendsen

fix temp/berendsen command
==========================

Syntax
""""""

.. parsed-literal::

   fix ID group-ID temp/berendsen Tstart Tstop Tdamp

* ID, group-ID are documented in :doc:`fix <fix>` command
* temp/berendsen = style name of this fix command
* Tstart,Tstop = desired temperature at start/end of run

  .. parsed-literal::

       Tstart can be a variable (see below)

* Tdamp = temperature damping parameter (time units)

Examples
""""""""

.. code-block:: LAMMPS

   fix 1 all temp/berendsen 300.0 300.0 100.0

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

Reset the temperature of a group of atoms by using a Berendsen
thermostat :ref:`(Berendsen) <Berendsen2>`, which rescales their velocities
every timestep.

The thermostat is applied to only the translational degrees of freedom
for the particles, which is an important consideration for finite-size
particles which have rotational degrees of freedom are being
thermostatted with this fix.  The translational degrees of freedom can
also have a bias velocity removed from them before thermostatting
takes place; see the description below.

The desired temperature at each timestep is a ramped value during the
run from *Tstart* to *Tstop*\ .  The *Tdamp* 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 fs or ps - see the
:doc:`units <units>` command).

*Tstart* can be specified as an equal-style :doc:`variable <variable>`.
In this case, the *Tstop* setting is ignored.  If the value is a
variable, it should be specified as v_name, where name is the variable
name.  In this case, the variable will be evaluated each timestep, and
its value used to determine the target temperature.

.. note::

   This thermostat will generate an error if the current
   temperature is zero at the end of a timestep.  It cannot rescale a
   zero temperature.

Equal-style variables can specify formulas with various mathematical
functions, and include :doc:`thermo_style <thermo_style>` command
keywords for the simulation box parameters and timestep and elapsed
time.  Thus it is easy to specify a time-dependent temperature.

.. note::

   Unlike the :doc:`fix nvt <fix_nh>` command which performs
   Nose/Hoover thermostatting AND time integration, this fix does NOT
   perform time integration.  It only modifies velocities to effect
   thermostatting.  Thus you must use a separate time integration fix,
   like :doc:`fix nve <fix_nve>` to actually update the positions of atoms
   using the modified velocities.  Likewise, this fix should not normally
   be used on atoms that also have their temperature controlled by
   another fix - e.g. by :doc:`fix nvt <fix_nh>` or :doc:`fix langevin <fix_langevin>` commands.

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

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:

.. code-block:: LAMMPS

   compute fix-ID_temp group-ID temp

See the :doc:`compute temp <compute_temp>` command for details.  Note
that the ID of the new compute is the fix-ID + underscore + "temp",
and the group for the new compute is the same as the fix group.

Note that this is NOT the compute used by thermodynamic output (see
the :doc:`thermo_style <thermo_style>` command) with ID = *thermo_temp*.
This means you can change the attributes of this fix's temperature
(e.g. its degrees-of-freedom) via the
:doc:`compute_modify <compute_modify>` command or print this temperature
during thermodynamic output via the :doc:`thermo_style custom <thermo_style>` command using the appropriate compute-ID.
It also means that changing attributes of *thermo_temp* will have no
effect on this fix.

Like other fixes that perform thermostatting, this fix can be used
with :doc:`compute commands <compute>` that calculate a temperature
after removing a "bias" from the atom velocities.  E.g. removing the
center-of-mass velocity from a group of atoms or only calculating
temperature on the x-component of velocity or only calculating
temperature for atoms in a geometric region.  This is not done by
default, but only if the :doc:`fix_modify <fix_modify>` command is used
to assign a temperature compute to this fix that includes such a bias
term.  See the doc pages for individual :doc:`compute commands <compute>` to determine which ones include a bias.  In
this case, the thermostat works in the following manner: the current
temperature is calculated taking the bias into account, bias is
removed from each atom, thermostatting is performed on the remaining
thermal degrees of freedom, and the bias is added back in.

----------

Restart, fix_modify, output, run start/stop, minimize info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

This fix writes the cumulative global energy change to
:doc:`binary restart files <restart>`.  See the
:doc:`read_restart <read_restart>` command for info on how to
re-specify a fix in an input script that reads a restart file,
so that the fix continues in an uninterrupted fashion.

The :doc:`fix_modify <fix_modify>` *temp* option is supported by this
fix.  You can use it to assign a temperature :doc:`compute <compute>`
you have defined to this fix which will be used in its thermostatting
procedure, as described above.  For consistency, the group used by
this fix and by the compute should be the same.

The cumulative energy change in the system imposed by this fix is
included in the :doc:`thermodynamic output <thermo_style>` keywords
*ecouple* and *econserve*.  See the :doc:`thermo_style <thermo_style>`
doc page for details.

This fix computes a global scalar which can be accessed by various
:doc:`output commands <Howto_output>`.  The scalar is the same
cumulative energy change due to this fix described in the previous
paragraph.  The scalar value calculated by this fix is "extensive".

This fix can ramp its target temperature over multiple runs, using the
*start* and *stop* keywords of the :doc:`run <run>` command.  See the
:doc:`run <run>` command for details of how to do this.

This fix is not invoked during :doc:`energy minimization <minimize>`.

Restrictions
""""""""""""

This fix can be used with dynamic groups as defined by the
:doc:`group <group>` command.  Likewise it can be used with groups to
which atoms are added or deleted over time, e.g. a deposition
simulation.  However, the conservation properties of the thermostat
and barostat are defined for systems with a static set of atoms.  You
may observe odd behavior if the atoms in a group vary dramatically
over time or the atom count becomes very small.

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

:doc:`fix nve <fix_nve>`, :doc:`fix nvt <fix_nh>`, :doc:`fix temp/rescale <fix_temp_rescale>`, :doc:`fix langevin <fix_langevin>`,
:doc:`fix_modify <fix_modify>`, :doc:`compute temp <compute_temp>`,
:doc:`fix press/berendsen <fix_press_berendsen>`

Default
"""""""

none

----------

.. _Berendsen2:

**(Berendsen)** Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem
Phys, 81, 3684 (1984).
