forked from lijiext/lammps
156 lines
4.6 KiB
Plaintext
156 lines
4.6 KiB
Plaintext
.. index:: pair_style lj/cubic
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pair_style lj/cubic command
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===========================
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pair_style lj/cubic/gpu command
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===============================
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pair_style lj/cubic/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 lj/cubic
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Examples
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""""""""
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.. parsed-literal::
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pair_style lj/cubic
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pair_coeff * * 1.0 0.8908987
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Description
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"""""""""""
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The *lj/cubic* style computes a truncated LJ interaction potential
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whose energy and force are continuous everywhere. Inside the
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inflection point the interaction is identical to the standard 12/6
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:doc:`Lennard-Jones <pair_lj>` potential. The LJ function outside the
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inflection point is replaced with a cubic function of distance. The
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energy, force, and second derivative are continuous at the inflection
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point. The cubic coefficient A3 is chosen so that both energy and
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force go to zero at the cutoff distance. Outside the cutoff distance
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the energy and force are zero.
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.. image:: Eqs/pair_lj_cubic.jpg
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:align: center
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The location of the inflection point rs is defined
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by the LJ diameter, rs/sigma = (26/7)^1/6. The cutoff distance
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is defined by rc/rs = 67/48 or rc/sigma = 1.737....
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The analytic expression for the
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the cubic coefficient
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A3*rmin^3/epsilon = 27.93... is given in the paper by
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Holian and Ravelo :ref:`(Holian) <Holian>`.
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This potential is commonly used to study the shock mechanics of FCC
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solids, as in Ravelo et al. :ref:`(Ravelo) <Ravelo>`.
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The following coefficients must be defined for each pair of atom types
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via the :doc:`pair_coeff <pair_coeff>` command as in the example above,
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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, or by mixing as described below:
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* epsilon (energy units)
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* sigma (distance units)
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Note that sigma is defined in the LJ formula as the zero-crossing
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distance for the potential, not as the energy minimum, which is
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located at rmin = 2^(1/6)*sigma. In the above example, sigma =
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0.8908987, so rmin = 1.
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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 in :doc:`Section_accelerate <Section_accelerate>`
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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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 :ref:`Making LAMMPS <start_3>` section 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 :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
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use the :doc:`suffix <suffix>` command in your input script.
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See :doc:`Section_accelerate <Section_accelerate>` of the manual for
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more 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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For atom type pairs I,J and I != J, the epsilon and sigma coefficients
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and cutoff distance for all of the lj/cut pair styles can be mixed.
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The default mix value is *geometric*\ . See the "pair_modify" command
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for details.
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The lj/cubic pair style does not support the
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:doc:`pair_modify <pair_modify>` shift option,
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since pair interaction is already smoothed to 0.0 at the
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cutoff.
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The :doc:`pair_modify <pair_modify>` table option is not relevant
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for this pair style.
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The lj/cubic pair style does not support the
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:doc:`pair_modify <pair_modify>` tail option for adding long-range tail
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corrections to energy and pressure, since there are no corrections for
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a potential that goes to 0.0 at the cutoff.
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The lj/cubic pair style writes its information to :doc:`binary restart files <restart>`, so pair_style and pair_coeff commands do
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not need to be specified in an input script that reads a restart file.
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The lj/cubic pair style can only be used via the *pair*
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keyword of the :doc:`run_style respa <run_style>` command. It does not
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support the *inner*\ , *middle*\ , *outer* keywords.
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----------
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Restrictions
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""""""""""""
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none
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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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.. _Holian:
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.. _Ravelo:
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**(Holian)** Holian and Ravelo, Phys Rev B, 51, 11275 (1995).
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**(Ravelo)** Ravelo, Holian, Germann and Lomdahl, Phys Rev B, 70, 014103 (2004).
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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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