39ea6ef30e
use the (newly redefined) \AA macro for Angstrom symbol throughout
instead of either \mathring{A} or AA
141 lines
4.8 KiB
ReStructuredText
141 lines
4.8 KiB
ReStructuredText
.. index:: fix acks2/reaxff
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.. index:: fix acks2/reaxff/kk
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fix acks2/reaxff command
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========================
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Accelerator Variants: *acks2/reaxff/kk*
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Syntax
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""""""
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.. code-block:: LAMMPS
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fix ID group-ID acks2/reaxff Nevery cutlo cuthi tolerance params args
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* acks2/reaxff = style name of this fix command
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* Nevery = perform ACKS2 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 = reaxff or a filename
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* one or more keywords or keyword/value pairs may be appended
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.. parsed-literal::
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keyword = *maxiter*
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*maxiter* N = limit the number of iterations to *N*
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 1 all acks2/reaxff 1 0.0 10.0 1.0e-6 reaxff
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fix 1 all acks2/reaxff 1 0.0 10.0 1.0e-6 param.acks2 maxiter 500
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Description
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"""""""""""
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Perform the atom-condensed Kohn--Sham DFT to second order (ACKS2) charge
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equilibration method as described in :ref:`(Verstraelen) <Verstraelen>`.
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ACKS2 impedes unphysical long-range charge transfer sometimes seen with
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QEq (e.g., for dissociation of molecules), at increased computational
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cost. It is typically used in conjunction with the ReaxFF force field
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model as implemented in the :doc:`pair_style reaxff <pair_reaxff>`
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command, but it can be used with any potential in LAMMPS, so long as it
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defines and uses charges on each atom. For more technical details about
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the charge equilibration performed by fix acks2/reaxff, see the
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:ref:`(O'Hearn) <O'Hearn>` paper.
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The ACKS2 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 requires some parameters for each atom type.
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If the *params* setting above is the word "reaxff", then these are
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extracted from the :doc:`pair_style reaxff <pair_reaxff>` 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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The lines should be formatted as follows:
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.. parsed-literal::
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bond_softness
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itype chi eta gamma bcut
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where the first line is the global parameter *bond_softness*. The
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remaining 1 to Ntypes lines include *itype*, the atom type from 1 to
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Ntypes, *chi*, the electronegativity in eV, *eta*, the self-Coulomb
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potential in eV, *gamma*, the valence orbital exponent, and *bcut*, the
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bond cutoff distance. Note that these 4 quantities are also in the
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ReaxFF potential file, except that eta is defined here as twice the eta
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value 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 :math:`\AA`, eV, and
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electronic charge.
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The optional *maxiter* keyword allows changing the max number
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of iterations in the linear solver. The default value is 200.
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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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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>`. This fix computes a global scalar (the number of
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iterations) 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.
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This fix is invoked during :doc:`energy minimization <minimize>`.
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----------
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.. include:: accel_styles.rst
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----------
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Restrictions
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""""""""""""
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This fix is part of the REAXFF package. It is 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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This fix does not correctly handle interactions involving multiple
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periodic images of the same atom. Hence, it should not be used for
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periodic cell dimensions less than :math:`10~\AA`.
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This fix may be used in combination with :doc:`fix efield <fix_efield>`
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and will apply the external electric field during charge equilibration,
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but there may be only one fix efield instance used, it may only use a
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constant electric field, and the electric field vector may only have
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components in non-periodic directions.
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Related commands
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""""""""""""""""
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:doc:`pair_style reaxff <pair_reaxff>`, :doc:`fix qeq/reaxff <fix_qeq_reaxff>`
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Default
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"""""""
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maxiter 200
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----------
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.. _O'Hearn:
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**(O'Hearn)** O'Hearn, Alperen, Aktulga, SIAM J. Sci. Comput., 42(1), C1--C22 (2020).
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.. _Verstraelen:
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**(Verstraelen)** Verstraelen, Ayers, Speybroeck, Waroquier, J. Chem. Phys. 138, 074108 (2013).
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