.. index:: fix mvv/dpd

fix mvv/dpd command
===================

fix mvv/edpd command
====================

fix mvv/tdpd command
====================

Syntax
""""""

.. parsed-literal::

   fix ID group-ID mvv/dpd lambda

   fix ID group-ID mvv/edpd lambda

   fix ID group-ID mvv/tdpd lambda

* ID, group-ID are documented in :doc:`fix <fix>` command
* mvv/dpd, mvv/edpd, mvv/tdpd = style name of this fix command
* lambda = (optional) relaxation parameter (unitless)

Examples
""""""""

.. code-block:: LAMMPS

   fix 1 all mvv/dpd
   fix 1 all mvv/dpd 0.5
   fix 1 all mvv/edpd
   fix 1 all mvv/edpd 0.5
   fix 1 all mvv/tdpd
   fix 1 all mvv/tdpd 0.5

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

Perform time integration using the modified velocity-Verlet (MVV)
algorithm to update position and velocity (fix mvv/dpd), or position,
velocity and temperature (fix mvv/edpd), or position, velocity and
concentration (fix mvv/tdpd) for particles in the group each timestep.

The modified velocity-Verlet (MVV) algorithm aims to improve the
stability of the time integrator by using an extrapolated version of
the velocity for the force evaluation:

.. math::

   v(t+\frac{\Delta t}{2}) = & v(t) + \frac{\Delta t}{2}\cdot a(t) \\
   r(t+\Delta t) = & r(t) + \Delta t\cdot v(t+\frac{\Delta t}{2}) \\
   a(t+\Delta t) = & \frac{1}{m}\cdot F\left[ r(t+\Delta t), v(t) +\lambda \cdot \Delta t\cdot a(t)\right] \\
   v(t+\Delta t) = & v(t+\frac{\Delta t}{2}) + \frac{\Delta t}{2}\cdot a(t+\Delta t)

where the parameter :math:`\lambda` depends on the
specific choice of DPD parameters, and needs to be tuned on a
case-by-case basis.  Specification of a *lambda* value is optional.
If specified, the setting must be from 0.0 to 1.0.  If not specified,
a default value of 0.5 is used, which effectively reproduces the
standard velocity-Verlet (VV) scheme.  For more details, see
:ref:`Groot <Groot2>`.

Fix *mvv/dpd* updates the position and velocity of each atom.  It can
be used with the :doc:`pair_style mdpd <pair_mesodpd>` command or other
pair styles such as :doc:`pair dpd <pair_dpd>`.

Fix *mvv/edpd* updates the per-atom temperature, in addition to
position and velocity, and must be used with the :doc:`pair_style edpd <pair_mesodpd>` command.

Fix *mvv/tdpd* updates the per-atom chemical concentration, in
addition to position and velocity, and must be used with the
:doc:`pair_style tdpd <pair_mesodpd>` command.

----------

**Restart, fix_modify, output, run start/stop, minimize info:**

No information about this fix is written to :doc:`binary restart files <restart>`.  None of the :doc:`fix_modify <fix_modify>` options
are relevant to this fix.  No global or per-atom quantities are stored
by this fix for access by various :doc:`output commands <Howto_output>`.
No parameter of this fix can be used with the *start/stop* keywords of
the :doc:`run <run>` command.  This fix is not invoked during :doc:`energy minimization <minimize>`.

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

This fix is part of the USER-MESODPD package. It is only enabled if
LAMMPS was built with that package. See the :doc:`Build package <Build_package>` doc page for more info.

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

:doc:`pair_style mdpd <pair_mesodpd>`, :doc:`pair_style edpd <pair_mesodpd>`,
:doc:`pair_style tdpd <pair_mesodpd>`

Default
"""""""

The default value for the optional *lambda* parameter is 0.5.

----------

.. _Groot2:

**(Groot)** Groot and Warren, J Chem Phys, 107: 4423-4435 (1997).  DOI:
10.1063/1.474784
