168 lines
5.8 KiB
ReStructuredText
168 lines
5.8 KiB
ReStructuredText
.. index:: pair_style buck/long/coul/long
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.. index:: pair_style buck/long/coul/long/omp
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pair_style buck/long/coul/long command
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======================================
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Accelerator Variants: *buck/long/coul/long/omp*
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Syntax
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""""""
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.. code-block:: LAMMPS
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pair_style buck/long/coul/long flag_buck flag_coul cutoff (cutoff2)
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* flag_buck = *long* or *cut*
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.. parsed-literal::
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*long* = use Kspace long-range summation for the dispersion term 1/r\^6
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*cut* = use a cutoff
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* flag_coul = *long* or *off*
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.. parsed-literal::
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*long* = use Kspace long-range summation for the Coulombic term 1/r
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*off* = omit the Coulombic term
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* cutoff = global cutoff for Buckingham (and Coulombic if only 1 cutoff) (distance units)
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* cutoff2 = global cutoff for Coulombic (optional) (distance units)
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Examples
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""""""""
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.. code-block:: LAMMPS
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pair_style buck/long/coul/long cut off 2.5
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pair_style buck/long/coul/long cut long 2.5 4.0
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pair_style buck/long/coul/long long long 4.0
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pair_coeff * * 1 1
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pair_coeff 1 1 1 3 4
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Description
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"""""""""""
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The *buck/long/coul/long* style computes a Buckingham potential (exp/6
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instead of Lennard-Jones 12/6) and Coulombic potential, given by
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.. math::
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E = & A e^{-r / \rho} - \frac{C}{r^6} \qquad r < r_c \\
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E = & \frac{C q_i q_j}{\epsilon r} \qquad r < r_c
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:math:`r_c` is the cutoff. If one cutoff is specified in the pair_style
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command, it is used for both the Buckingham and Coulombic terms. If
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two cutoffs are specified, they are used as cutoffs for the Buckingham
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and Coulombic terms respectively.
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The purpose of this pair style is to capture long-range interactions
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resulting from both attractive 1/r\^6 Buckingham and Coulombic 1/r
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interactions. This is done by use of the *flag_buck* and *flag_coul*
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settings. The :ref:`Ismail <Ismail>` paper has more details on when it is
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appropriate to include long-range 1/r\^6 interactions, using this
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potential.
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If *flag_buck* is set to *long*, no cutoff is used on the Buckingham
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1/r\^6 dispersion term. The long-range portion can be calculated by
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using the :doc:`kspace_style ewald/disp or pppm/disp <kspace_style>`
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commands. The specified Buckingham cutoff then determines which
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portion of the Buckingham interactions are computed directly by the
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pair potential versus which part is computed in reciprocal space via
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the Kspace style. If *flag_buck* is set to *cut*, the Buckingham
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interactions are simply cutoff, as with :doc:`pair_style buck <pair_buck>`.
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If *flag_coul* is set to *long*, no cutoff is used on the Coulombic
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interactions. The long-range portion can calculated by using any of
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several :doc:`kspace_style <kspace_style>` command options such as
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*pppm* or *ewald*\ . Note that if *flag_buck* is also set to long, then
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the *ewald/disp* or *pppm/disp* Kspace style needs to be used to
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perform the long-range calculations for both the Buckingham and
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Coulombic interactions. If *flag_coul* is set to *off*, Coulombic
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interactions are not computed.
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The following coefficients must be defined for each pair of atoms
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types via the :doc:`pair_coeff <pair_coeff>` command as in the examples
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above, 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 (energy units)
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* rho (distance units)
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* C (energy-distance\^6 units)
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* cutoff (distance units)
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* cutoff2 (distance units)
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The second coefficient, rho, must be greater than zero.
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The latter 2 coefficients are optional. If not specified, the global
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Buckingham and Coulombic cutoffs specified in the pair_style command
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are used. If only one cutoff is specified, it is used as the cutoff
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for both Buckingham and Coulombic interactions for this type pair. If
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both coefficients are specified, they are used as the Buckingham and
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Coulombic cutoffs for this type pair. Note that if you are using
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*flag_buck* set to *long*, you cannot specify a Buckingham cutoff for
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an atom type pair, since only one global Buckingham cutoff is allowed.
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Similarly, if you are using *flag_coul* set to *long*, you cannot
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specify a Coulombic cutoff for an atom type pair, since only one
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global Coulombic cutoff is allowed.
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----------
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.. include:: accel_styles.rst
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----------
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Mixing, shift, table, tail correction, restart, rRESPA info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This pair style does not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.
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This pair style supports the :doc:`pair_modify <pair_modify>` shift
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option for the energy of the exp() and 1/r\^6 portion of the pair
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interaction, assuming *flag_buck* is *cut*\ .
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This pair style does not support the :doc:`pair_modify <pair_modify>`
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shift option for the energy of the Buckingham portion of the pair
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interaction.
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This pair style supports the :doc:`pair_modify <pair_modify>` table and
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table/disp options since they can tabulate the short-range portion of
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the long-range Coulombic and dispersion interactions.
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This pair style write its information to :doc:`binary restart files <restart>`, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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This pair style supports the use of the *inner*, *middle*, and *outer*
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keywords of the :doc:`run_style respa <run_style>` command, meaning the
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pairwise forces can be partitioned by distance at different levels of
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the rRESPA hierarchy. See the :doc:`run_style <run_style>` command for
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details.
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----------
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Restrictions
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""""""""""""
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This style is part of the KSPACE package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`pair_coeff <pair_coeff>`
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Default
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"""""""
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none
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----------
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.. _Ismail:
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**(Ismail)** Ismail, Tsige, In 't Veld, Grest, Molecular Physics
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(accepted) (2007).
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