Homec4science
Diffusion lammps (master) Browse

lammps/doc/html/_sources/pair_dpd_fdt.txt
076464e41187master

Last Change
Raw File

pair_dpd_fdt.txt
View Options

.. index:: pair_style dpd/fdt
pair_style dpd/fdt command
==========================
pair_style dpd/fdt/energy command
=================================
Syntax
""""""
.. parsed-literal::
pair_style style args
* style = *dpd/fdt* or *dpd/fdt/energy*
* args = list of arguments for a particular style
.. parsed-literal::
*dpd/fdt* args = T cutoff seed
T = temperature (temperature units)
cutoff = global cutoff for DPD interactions (distance units)
seed = random # seed (positive integer)
*dpd/fdt/energy* args = cutoff seed
cutoff = global cutoff for DPD interactions (distance units)
seed = random # seed (positive integer)
Examples
""""""""
.. parsed-literal::
pair_style dpd/fdt 300.0 2.5 34387
pair_coeff * * 3.0 1.0 2.5
.. parsed-literal::
pair_style dpd/fdt/energy 2.5 34387
pair_coeff * * 3.0 1.0 0.1 2.5
Description
"""""""""""
Styles *dpd/fdt* and *dpd/fdt/energy* set the fluctuation-dissipation
theorem parameters and compute the conservative force for dissipative
particle dynamics (DPD). The conservative force on atom I due to atom
J is given by
.. image:: Eqs/pair_dpd_conservative.jpg
:align: center
where the weighting factor, omega_ij, varies between 0 and 1, and is
chosen to have the following functional form:
.. image:: Eqs/pair_dpd_omega.jpg
:align: center
where Rij is a unit vector in the direction Ri - Rj, and Rc is the
cutoff. Note that alternative definitions of the weighting function
exist, but would have to be implemented as a separate pair style
command.
These pair style differ from the other dpd styles in that the
dissipative and random forces are not computed within the pair style.
This style can be combined with the :doc:`fix shardlow <fix_shardlow>`
to perform the stochastic integration of the dissipative and random
forces through the Shardlow splitting algorithm approach.
The pairwise energy associated with styles *dpd/fdt* and
*dpd/fdt/energy* is only due to the conservative force term Fc, and is
shifted to be zero at the cutoff distance Rc. The pairwise virial is
calculated using only the conservative term.
For style *dpd/fdt*\ , the fluctuation-dissipation theorem defines gamma
to be set equal to sigma*sigma/(2 T), where T is the set point
temperature specified as a pair style parameter in the above examples.
This style can be combined with :doc:`fix shardlow <fix_shardlow>` to
perform DPD simulations under isothermal and isobaric conditions (see
:ref:`(Lisal) <Lisal>`). The following coefficients must be defined for
each pair of atoms types via the :doc:`pair_coeff <pair_coeff>` command
as in the examples above, or in the data file or restart files read by
the :doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands:
* A (force units)
* sigma (force*time^(1/2) units)
* cutoff (distance units)
The last coefficient is optional. If not specified, the global DPD
cutoff is used.
For style *dpd/fdt/energy*\ , the fluctuation-dissipation theorem
defines gamma to be set equal to sigma*sigma/(2 dpdTheta), where
dpdTheta is the average internal temperature for the pair.
Furthermore, the fluctuation-dissipation defines alpha*alpha to be set
equal to 2*kB*kappa, where kappa is the mesoparticle thermal
conductivity parameter. This style can be combined with :doc:`fix shardlow <fix_shardlow>` to perform DPD simulations under
isoenergetic and isoenthalpic conditions (see :ref:`(Lisal) <Lisal>`). The
following coefficients must be defined for each pair of atoms types
via the :doc:`pair_coeff <pair_coeff>` command as in the examples above,
or in the data file or restart files read by the
:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands:
* A (force units)
* sigma (force*time^(1/2) units)
* kappa (1/time units)
* cutoff (distance units)
The last coefficient is optional. If not specified, the global DPD
cutoff is used.
For style *dpd/fdt/energy*\ , the particle internal temperature is
related to the particle internal energy through a mesoparticle
equation of state. Thus, an an additional :doc:`fix eos <fix>` must be
specified.
----------
Restrictions
""""""""""""
These commands are part of the USER-DPD package. They are only
enabled if LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
Pair styles *dpd/fdt* and *dpd/fdt/energy* require use of the
:doc:`communicate vel yes <communicate>` option so that velocites are
stored by ghost atoms.
Pair style *dpd/fdt/energy* requires :doc:`atom_style dpd <atom_style>`
to be used in order to properly account for the particle internal
energies and temperatures.
Related commands
""""""""""""""""
:doc:`pair_coeff <pair_coeff>`, :doc:`fix shardlow <fix_shardlow>`
**Default:** none
----------
.. _Lisal:
**(Lisal)** M. Lisal, J.K. Brennan, J. Bonet Avalos, "Dissipative
particle dynamics as isothermal, isobaric, isoenergetic, and
isoenthalpic conditions using Shardlow-like splitting algorithms.",
J. Chem. Phys., 135, 204105 (2011).
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
c4science · Help