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