forked from lijiext/lammps
118 lines
3.7 KiB
Plaintext
118 lines
3.7 KiB
Plaintext
.. index:: fix qeq/reax
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fix qeq/reax command
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====================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID qeq/reax Nevery cutlo cuthi tolerance params
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* qeq/reax = style name of this fix command
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* Nevery = perform QEq every this many steps
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* cutlo,cuthi = lo and hi cutoff for Taper radius
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* tolerance = precision to which charges will be equilibrated
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* params = reax/c or a filename
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Examples
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""""""""
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.. parsed-literal::
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fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c
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fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 param.qeq
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Description
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"""""""""""
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Perform the charge equilibration (QEq) method as described in :ref:`(Rappe and Goddard) <Rappe>` and formulated in :ref:`(Nakano) <Nakano>`. It is
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typically used in conjunction with the ReaxFF force field model as
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implemented in the :doc:`pair_style reax/c <pair_reax_c>` command, but
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it can be used with any potential in LAMMPS, so long as it defines and
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uses charges on each atom. The :doc:`fix qeq/comb <fix_qeq_comb>`
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command should be used to perform charge equliibration with the :doc:`COMB potential <pair_comb>`. For more technical details about the
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charge equilibration performed by fix qeq/reax, see the
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:ref:`(Aktulga) <Aktulga>` paper.
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The QEq method minimizes the electrostatic energy of the system by
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adjusting the partial charge on individual atoms based on interactions
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with their neighbors. It reqires some parameters for each atom type.
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If the *params* setting above is the word "reax/c", then these are
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extracted from the :doc:`pair_style reax/c <pair_reax_c>` command and
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the ReaxFF force field file it reads in. If a file name is specified
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for *params*\ , then the parameters are taken from the specified file
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and the file must contain one line for each atom type. The latter
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form must be used when performing QeQ with a non-ReaxFF potential.
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Each line should be formatted as follows:
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.. parsed-literal::
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itype chi eta gamma
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where *itype* is the atom type from 1 to Ntypes, *chi* denotes the
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electronegativity in eV, *eta* denotes the self-Coulomb
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potential in eV, and *gamma* denotes the valence orbital
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exponent. Note that these 3 quantities are also in the ReaxFF
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potential file, except that eta is defined here as twice the eta value
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in the ReaxFF file. Note that unlike the rest of LAMMPS, the units
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of this fix are hard-coded to be A, eV, and electronic charge.
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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 <restart>`. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various :ref:`output commands <howto_15>`. No parameter of this fix can
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be used with the *start/stop* keywords of the :doc:`run <run>` command.
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This fix is invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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This fix is part of the USER-REAXC package. It is only enabled if
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LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
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This fix does not correctly handle interactions
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involving multiple periodic images of the same atom. Hence, it should not
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be used for periodic cell dimensions less than 10 angstroms.
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Related commands
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""""""""""""""""
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:doc:`pair_style reax/c <pair_reax_c>`
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**Default:** none
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----------
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.. _Rappe:
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**(Rappe)** Rappe and Goddard III, Journal of Physical Chemistry, 95,
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3358-3363 (1991).
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.. _Nakano:
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**(Nakano)** Nakano, Computer Physics Communications, 104, 59-69 (1997).
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.. _Aktulga:
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(Aktulga) Aktulga, Fogarty, Pandit, Grama, Parallel Computing, 38,
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245-259 (2012).
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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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