200 lines
6.8 KiB
ReStructuredText
200 lines
6.8 KiB
ReStructuredText
.. index:: pair\_style buck/long/coul/long
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pair\_style buck/long/coul/long command
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=======================================
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pair\_style buck/long/coul/long/omp 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 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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.. parsed-literal::
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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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.. image:: Eqs/pair_buck.jpg
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:align: center
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.. image:: Eqs/pair_coulomb.jpg
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:align: center
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Rc 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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Styles with a *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed on the :doc:`Speed packages <Speed_packages>` doc
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page. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
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USER-OMP and OPT packages, respectively. They are only enabled if
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LAMMPS was built with those packages. See the :doc:`Build package <Build_package>` doc page for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the :doc:`-suffix command-line switch <Run_options>` when you invoke LAMMPS, or you can use the
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:doc:`suffix <suffix>` command in your input script.
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See the :doc:`Speed packages <Speed_packages>` doc page for more
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instructions on how to use the accelerated styles effectively.
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----------
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**Mixing, shift, table, tail correction, restart, rRESPA info**\ :
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This pair styles 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>` doc 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:** 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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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Commands_all.html
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