135 lines
4.1 KiB
ReStructuredText
135 lines
4.1 KiB
ReStructuredText
.. index:: fix qeq/comb
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.. index:: fix qeq/comb/omp
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fix qeq/comb command
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====================
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Accelerator Variants: *qeq/comb/omp*
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID qeq/comb Nevery precision keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* qeq/comb = style name of this fix command
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* Nevery = perform charge equilibration every this many steps
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* precision = convergence criterion for charge equilibration
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* zero or more keyword/value pairs may be appended
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* keyword = *file*
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.. parsed-literal::
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*file* value = filename
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filename = name of file to write QEQ equilibration info to
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 1 surface qeq/comb 10 0.0001
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Description
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"""""""""""
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Perform charge equilibration (QeQ) in conjunction with the COMB
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(Charge-Optimized Many-Body) potential as described in
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:ref:`(COMB_1) <COMB_1>` and :ref:`(COMB_2) <COMB_2>`. It performs the charge
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equilibration portion of the calculation using the so-called QEq
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method, whereby the charge on each atom is adjusted to minimize the
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energy of the system. This fix can only be used with the COMB
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potential; see the :doc:`fix qeq/reaxff <fix_qeq_reaxff>` command for a QeQ
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calculation that can be used with any potential.
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Only charges on the atoms in the specified group are equilibrated.
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The fix relies on the pair style (COMB in this case) to calculate the
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per-atom electronegativity (effective force on the charges). An
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electronegativity equalization calculation (or QEq) is performed in an
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iterative fashion, which in parallel requires communication at each
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iteration for processors to exchange charge information about nearby
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atoms with each other. See :ref:`Rappe_and_Goddard <Rappe_and_Goddard>` and
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:ref:`Rick_and_Stuart <Rick_and_Stuart>` for details.
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During a run, charge equilibration is performed every *Nevery* time
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steps. Charge equilibration is also always enforced on the first step
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of each run. The *precision* argument controls the tolerance for the
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difference in electronegativity for all atoms during charge
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equilibration. *Precision* is a trade-off between the cost of
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performing charge equilibration (more iterations) and accuracy.
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If the *file* keyword is used, then information about each
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equilibration calculation is written to the specified file.
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.. note::
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In order to solve the self-consistent equations for electronegativity
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equalization, LAMMPS imposes the additional constraint that all the
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charges in the fix group must add up to zero. The initial charge
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assignments should also satisfy this constraint. LAMMPS will print a
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warning if that is not the case.
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----------
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.. include:: accel_styles.rst
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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 <restart>`.
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The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
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fix. This allows to set at which level of the :doc:`r-RESPA <run_style>`
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integrator the fix is performing charge equilibration. Default is
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the outermost level.
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This fix produces a per-atom vector which can be accessed by various
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:doc:`output commands <Howto_output>`. The vector stores the gradient
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of the charge on each atom. The per-atom values be accessed on any
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timestep.
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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.
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This fix can be invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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This fix command currently only supports :doc:`pair style *comb*\ <pair_comb>`.
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Related commands
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""""""""""""""""
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:doc:`pair_style comb <pair_comb>`
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Default
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"""""""
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No file output is performed.
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----------
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.. _COMB_1:
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**(COMB_1)** J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),
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.. _COMB_2:
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**(COMB_2)** T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R.
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Phillpot, Phys Rev B, 81, 125328 (2010).
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.. _Rappe_and_Goddard:
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**(Rappe_and_Goddard)** A. K. Rappe, W. A. Goddard, J Phys Chem 95, 3358
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(1991).
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.. _Rick_and_Stuart:
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**(Rick_and_Stuart)** S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys
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101, 16141 (1994).
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