135 lines
4.1 KiB
ReStructuredText
135 lines
4.1 KiB
ReStructuredText
.. index:: pair_style soft
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.. index:: pair_style soft/gpu
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.. index:: pair_style soft/kk
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.. index:: pair_style soft/omp
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pair_style soft command
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=======================
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Accelerator Variants: *soft/gpu*, *soft/kk*, *soft/omp*
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Syntax
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""""""
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.. code-block:: LAMMPS
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pair_style soft cutoff
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* cutoff = global cutoff for soft interactions (distance units)
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Examples
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""""""""
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.. code-block:: LAMMPS
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pair_style soft 1.0
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pair_coeff * * 10.0
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pair_coeff 1 1 10.0 3.0
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pair_style soft 1.0
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pair_coeff * * 0.0
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variable prefactor equal ramp(0,30)
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fix 1 all adapt 1 pair soft a * * v_prefactor
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Description
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"""""""""""
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Style *soft* computes pairwise interactions with the formula
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.. math::
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E = A \left[ 1 + \cos\left(\frac{\pi r}{r_c}\right) \right]
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\qquad r < r_c
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It is useful for pushing apart overlapping atoms, since it does not
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blow up as r goes to 0. A is a prefactor that can be made to vary in
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time from the start to the end of the run (see discussion below),
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e.g. to start with a very soft potential and slowly harden the
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interactions over time. :math:`r_c` is the cutoff.
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See the :doc:`fix nve/limit <fix_nve_limit>` command for another way
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to push apart overlapping atoms.
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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 examples 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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* A (energy units)
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* cutoff (distance units)
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The last coefficient is optional. If not specified, the global soft
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cutoff is used.
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.. note::
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The syntax for :doc:`pair_coeff <pair_coeff>` with a single A
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coeff is different in the current version of LAMMPS than in older
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versions which took two values, Astart and Astop, to ramp between
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them. This functionality is now available in a more general form
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through the :doc:`fix adapt <fix_adapt>` command, as explained below.
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Note that if you use an old input script and specify Astart and Astop
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without a cutoff, then LAMMPS will interpret that as A and a cutoff,
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which is probably not what you want.
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The :doc:`fix adapt <fix_adapt>` command can be used to vary A for one
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or more pair types over the course of a simulation, in which case
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pair_coeff settings for A must still be specified, but will be
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overridden. For example these commands will vary the prefactor A for
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all pairwise interactions from 0.0 at the beginning to 30.0 at the end
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of a run:
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.. code-block:: LAMMPS
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variable prefactor equal ramp(0,30)
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fix 1 all adapt 1 pair soft a * * v_prefactor
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Note that a formula defined by an :doc:`equal-style variable <variable>`
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can use the current timestep, elapsed time in the current run, elapsed
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time since the beginning of a series of runs, as well as access other
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variables.
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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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For atom type pairs I,J and I != J, the A coefficient and cutoff
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distance for this pair style can be mixed. A is always mixed via a
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*geometric* rule. The cutoff is mixed according to the pair_modify
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mix value. The default mix value is *geometric*\ . See the
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"pair_modify" command for details.
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This pair style does not support the :doc:`pair_modify <pair_modify>`
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shift option, since the pair interaction goes to 0.0 at the cutoff.
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The :doc:`pair_modify <pair_modify>` table and tail options are not
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relevant for this pair style.
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This pair style writes 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 can only be used via the *pair* keyword of the
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:doc:`run_style respa <run_style>` command. It does not support the
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*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>`, :doc:`fix nve/limit <fix_nve_limit>`, :doc:`fix adapt <fix_adapt>`
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Default
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"""""""
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none
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