.. index:: fix nvt/sllod
.. index:: fix nvt/sllod/intel
.. index:: fix nvt/sllod/omp
.. index:: fix nvt/sllod/kk

fix nvt/sllod command
=====================

Accelerator Variants: *nvt/sllod/intel*, *nvt/sllod/omp*, *nvt/sllod/kk*

Syntax
""""""

.. code-block:: LAMMPS

   fix ID group-ID nvt/sllod keyword value ...

* ID, group-ID are documented in :doc:`fix <fix>` command
* nvt/sllod = style name of this fix command
* zero or more keyword/value pairs may be appended

  .. parsed-literal::

    keyword = *psllod*
      *psllod* value = *no* or *yes* = use SLLOD or p-SLLOD variant, respectively

* additional thermostat related keyword/value pairs from the :doc:`fix nvt <fix_nh>`
  command can be appended

Examples
""""""""

.. code-block:: LAMMPS

   fix 1 all nvt/sllod temp 300.0 300.0 100.0
   fix 1 all nvt/sllod temp 300.0 300.0 100.0 drag 0.2

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

Perform constant NVT integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover temperature
thermostat.  V is volume; T is temperature.  This creates a system
trajectory consistent with the canonical ensemble.

This thermostat is used for a simulation box that is changing size
and/or shape, for example in a non-equilibrium MD (NEMD) simulation.
The size/shape change is induced by use of the :doc:`fix deform
<fix_deform>` command, so each point in the simulation box can be
thought of as having a "streaming" velocity.  This position-dependent
streaming velocity is subtracted from each atom's actual velocity to
yield a thermal velocity which is used for temperature computation and
thermostatting.  For example, if the box is being sheared in x,
relative to y, then points at the bottom of the box (low y) have a
small x velocity, while points at the top of the box (hi y) have a
large x velocity.  These velocities do not contribute to the thermal
"temperature" of the atom.

.. note::

   :doc:`Fix deform <fix_deform>` has an option for remapping either
   atom coordinates or velocities to the changing simulation box.  To use
   fix nvt/sllod, fix deform should NOT remap atom positions, because fix
   nvt/sllod adjusts the atom positions and velocities to create a
   velocity profile that matches the changing box size/shape.  Fix deform
   SHOULD remap atom velocities when atoms cross periodic boundaries
   since that is consistent with maintaining the velocity profile created
   by fix nvt/sllod.  LAMMPS will give an error if this setting is not
   consistent.

The SLLOD equations of motion, originally proposed by Hoover and Ladd
(see :ref:`(Evans and Morriss) <Evans3>`), were proven to be equivalent
to Newton's equations of motion for shear flow by :ref:`(Evans and
Morriss) <Evans3>`. They were later shown to generate the desired
velocity gradient and the correct production of work by stresses for all
forms of homogeneous flow by :ref:`(Daivis and Todd) <Daivis>`.

.. versionchanged:: 8Feb2023

For the default (*psllod* = *no*), the LAMMPS implementation adheres to
the standard SLLOD equations of motion, as defined by :ref:`(Evans and
Morriss) <Evans3>`.  The option *psllod* = *yes* invokes the slightly
different SLLOD variant first introduced by :ref:`(Tuckerman et al.)
<Tuckerman>` as g-SLLOD and later by :ref:`(Edwards) <Edwards>` as
p-SLLOD.  In all cases, the equations of motion are coupled to a
Nose/Hoover chain thermostat in a velocity Verlet formulation, closely
following the implementation used for the :doc:`fix nvt <fix_nh>`
command.

.. note::

   A recent (2017) book by :ref:`(Todd and Daivis) <Todd-sllod>`
   discusses use of the SLLOD method and non-equilibrium MD (NEMD)
   thermostatting generally, for both simple and complex fluids,
   e.g. molecular systems.  The latter can be tricky to do correctly.

Additional parameters affecting the thermostat are specified by
keywords and values documented with the :doc:`fix nvt <fix_nh>`
command.  See, for example, discussion of the *temp* and *drag*
keywords.

This fix computes a temperature each timestep.  To do this, the fix
creates its own compute of style "temp/deform", as if this command had
been issued:

.. code-block:: LAMMPS

   compute fix-ID_temp group-ID temp/deform

See the :doc:`compute temp/deform <compute_temp_deform>` 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 remove a "bias" from the
atom velocities.  E.g. to apply the thermostat only to atoms within a
spatial :doc:`region <region>`, or to remove the center-of-mass
velocity from a group of atoms, or to remove the x-component of
velocity from the calculation.

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 temp commands <compute>` to determine which ones include
a bias.  In this case, the thermostat works in the following manner:
bias is removed from each atom, thermostatting is performed on the
remaining thermal degrees of freedom, and the bias is added back in.

----------

.. include:: accel_styles.rst

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

This fix writes the state of the Nose/Hoover thermostat 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 operation of 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 :doc:`compute <compute>` you have
defined to this fix which will be used in its thermostatting
procedure.

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 the same global scalar and global vector of
quantities as does the :doc:`fix nvt <fix_nh>` command.

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 works best without Nose-Hoover chain thermostats, i.e. using
*tchain* = 1.  Setting *tchain* to larger values can result in poor
equilibration.

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/deform <compute_temp_deform>`

Default
"""""""

Same as :doc:`fix nvt <fix_nh>`, except *tchain* = 1, psllod = *no*.

----------

.. _Evans3:

**(Evans and Morriss)** Evans and Morriss, Phys Rev A, 30, 1528 (1984).

.. _Daivis:

**(Daivis and Todd)** Daivis and Todd, J Chem Phys, 124, 194103 (2006).

.. _Todd-sllod:

**(Todd and Daivis)** Todd and Daivis, Nonequilibrium Molecular Dynamics (book),
Cambridge University Press, (2017) https://doi.org/10.1017/9781139017848.

.. _Tuckerman:

**(Tuckerman et al.)** Tuckerman, Mundy, Balasubramanian, and Klein, J Chem Phys 106, 5615 (1997).

.. _Edwards:

**(Edwards)** Edwards, Baig, and Keffer, J Chem Phys 124, 194104 (2006).

