Added the documentation

doc/src/pair_wf.rst

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+.. index:: pair_style wf
+
+pair_style wf command
+===========================
+
+Syntax
+""""""
+
+
+.. code-block:: LAMMPS
+
+   pair_style wf cutoff
+
+* cutoff = cutoff for wf interactions (distance units)
+
+Examples
+""""""""
+
+
+.. code-block:: LAMMPS
+
+   variable         sigma  equal   1.0
+   variable         epsilon equal 1.0
+   variable         nu   equal 1.0
+   variable         mu   equal 1.0
+   variable         rc equal 2.0*${sigma}
+
+   pair_style        wf ${rc}
+   pair_coeff        1 1 ${epsilon} ${sigma} ${nu} ${mu}  ${rc}	
+
+Description
+"""""""""""
+
+The *wf* style computes the potential in :ref:`Wang2020 <Wang2020>`, which is given by:
+
+.. math::
+
+  \phi(r)= \epsilon \alpha \left(\left[{\sigma\over r}\right]^{2\mu} -1 \right)\left(\left[{r_c\over r}\right]^{2\mu}-1\right)^{2\nu}
+
+with
+
+.. math::
+  \alpha=2\nu\left(\frac{r_c}{\sigma}\right)^{2\mu}\left[\frac{1+2\nu}{2\nu\left[(r_c/\sigma)^{2\mu}-1\right]}\right]^{2\nu+1}
+
+and
+
+.. math::
+  r_{min}=r_c\left[\frac{1+2\nu}{1+2\nu(r_c/\sigma)^{2\nu}}\right]^{1/{2\nu}}
+
+:math:`r_c` is the cutoff. 
+
+The following coefficients must be defined for each pair of atoms
+types via the :doc:`pair_coeff <pair_coeff>` command as in the example
+above, or in the data file or restart files read by the
+:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
+commands:
+
+* :math:`\epsilon` (energy units)
+* :math:`\sigma` (distance units)
+* :math:`\nu` 
+* :math:`\mu`
+* :math:`r_c` (distance units)
+
+----------
+
+
+Styles with a *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
+functionally the same as the corresponding style without the suffix.
+They have been optimized to run faster, depending on your available
+hardware, as discussed on the :doc:`Speed packages <Speed_packages>` doc
+page.  The accelerated styles take the same arguments and should
+produce the same results, except for round-off and precision issues.
+
+These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
+USER-OMP and OPT packages, respectively.  They are only enabled if
+LAMMPS was built with those packages.  See the :doc:`Build package <Build_package>` doc page for more info.
+
+You can specify the accelerated styles explicitly in your input script
+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
+:doc:`suffix <suffix>` command in your input script.
+
+See the :doc:`Speed packages <Speed_packages>` doc page for more
+instructions on how to use the accelerated styles effectively.
+
+
+----------
+
+
+**Mixing, shift, table, tail correction, restart, rRESPA info**\ :
+
+For atom type pairs I,J and I != J, the epsilon and sigma coefficients
+can be  mixed. Careful is required with the cut-off radius. 
+The default mix value is *geometric*\ .  See the "pair\_modify" command
+for details.
+
+The :doc:`pair_modify <pair_modify>` tail option is not relevant
+for this pair style as it goes to zero at the cut-off radius.
+
+
+This pair style writes its information to :doc:`binary restart files <restart>`, so pair\_style and pair\_coeff commands do not need
+to be specified in an input script that reads a restart file.
+
+This pair style does not support the use of the *inner*\ , *middle*\ , and *outer*
+keywords of the :doc:`run_style respa <run_style>` command.
+
+
+----------
+
+
+Restrictions
+""""""""""""
+none
+
+Related commands
+""""""""""""""""
+
+:doc:`pair_coeff <pair_coeff>`
+
+**Default:** none
+
+
+----------
+
+.. _Wang2020:
+
+**(Wang2020)** X. Wang, S. Ramírez-Hinestrosa, J. Dobnikar, and D. Frenkel, Phys. Chem. Chem. Phys. 22, 10624 (2020).