117 lines
3.5 KiB
ReStructuredText
117 lines
3.5 KiB
ReStructuredText
.. index:: fix mvv/dpd
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.. index:: fix mvv/edpd
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.. index:: fix mvv/tdpd
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fix mvv/dpd command
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===================
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fix mvv/edpd command
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====================
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fix mvv/tdpd command
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====================
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Syntax
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""""""
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.. code-block:: LAMMPS
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fix ID group-ID mvv/dpd lambda
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fix ID group-ID mvv/edpd lambda
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fix ID group-ID mvv/tdpd lambda
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* mvv/dpd, mvv/edpd, mvv/tdpd = style name of this fix command
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* lambda = (optional) relaxation parameter (unitless)
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 1 all mvv/dpd
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fix 1 all mvv/dpd 0.5
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fix 1 all mvv/edpd
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fix 1 all mvv/edpd 0.5
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fix 1 all mvv/tdpd
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fix 1 all mvv/tdpd 0.5
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Description
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"""""""""""
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Perform time integration using the modified velocity-Verlet (MVV)
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algorithm to update position and velocity (fix mvv/dpd), or position,
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velocity and temperature (fix mvv/edpd), or position, velocity and
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concentration (fix mvv/tdpd) for particles in the group each timestep.
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The modified velocity-Verlet (MVV) algorithm aims to improve the
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stability of the time integrator by using an extrapolated version of
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the velocity for the force evaluation:
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.. math::
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v(t+\frac{\Delta t}{2}) = & v(t) + \frac{\Delta t}{2}\cdot a(t) \\
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r(t+\Delta t) = & r(t) + \Delta t\cdot v(t+\frac{\Delta t}{2}) \\
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a(t+\Delta t) = & \frac{1}{m}\cdot F\left[ r(t+\Delta t), v(t) +\lambda \cdot \Delta t\cdot a(t)\right] \\
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v(t+\Delta t) = & v(t+\frac{\Delta t}{2}) + \frac{\Delta t}{2}\cdot a(t+\Delta t)
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where the parameter :math:`\lambda` depends on the
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specific choice of DPD parameters, and needs to be tuned on a
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case-by-case basis. Specification of a *lambda* value is optional.
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If specified, the setting must be from 0.0 to 1.0. If not specified,
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a default value of 0.5 is used, which effectively reproduces the
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standard velocity-Verlet (VV) scheme. For more details, see
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:ref:`Groot <Groot2>`.
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Fix *mvv/dpd* updates the position and velocity of each atom. It can be
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used with the :doc:`pair_style mdpd <pair_mesodpd>` command or other
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pair styles such as :doc:`pair dpd <pair_dpd>`.
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Fix *mvv/edpd* updates the per-atom temperature, in addition to position
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and velocity, and must be used with the :doc:`pair_style edpd
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<pair_mesodpd>` command.
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Fix *mvv/tdpd* updates the per-atom chemical concentration, in addition
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to position and velocity, and must be used with the :doc:`pair_style
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tdpd <pair_mesodpd>` command.
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files
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<restart>`. None of the :doc:`fix_modify <fix_modify>` options are
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relevant to this fix. No global or per-atom quantities are stored by
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this fix for access by various :doc:`output commands <Howto_output>`.
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command. This fix is not invoked during
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:doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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These fixes are part of the DPD-MESO package. They are only enabled if
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LAMMPS was built with that package. See the :doc:`Build package
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<Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`pair_style mdpd <pair_mesodpd>`, :doc:`pair_style edpd <pair_mesodpd>`,
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:doc:`pair_style tdpd <pair_mesodpd>`
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Default
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"""""""
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The default value for the optional *lambda* parameter is 0.5.
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----------
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.. _Groot2:
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**(Groot)** Groot and Warren, J Chem Phys, 107: 4423-4435 (1997). DOI:
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10.1063/1.474784
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