revised doc for bond_style special

The *special* bond style can be used to impose weighted Lennard Jones and/or

Coloumbic interactions on any two particles in the system. It can be used for

cases that cannot be handled in :doc:`special_bonds <special_bonds>`, such as

1-5 interactions or hyrbrid simulations that require the use of different

weighting factors for different molecules.

1-5 interactions or complex systems that require the use of different

weighting factors for different molecules. It is a potential of the form:

.. math::

   E =  w_{LJ} E_{LJ} + w_{Coul}E_{Coul}

The following coefficients must be defined for each bond type via the

:doc:`bond_coeff <bond_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:

* `w_lj` weight (0.0 to 1.0) on pairwise Lennard-Jones interactions

* :math:`w_{LJ}` weight (0.0 to 1.0) on pairwise Lennard-Jones interactions

* `w_coul` weight (0.0 to 1.0) on pairwise Coulombic interactions

This command will typically be used in conjunction with 

* :math:`w_{Coul}` weight (0.0 to 1.0) on pairwise Coulombic interactions

----------

This style has strict requirements on the :doc:`special_bonds <special_bonds>`

setting. 1-2 interactions must have weights of zero. 1-3 interactions must

either have weights of zero or the *angle* setting must be turned on. 1-4

interactions must have weights of zero or the *dihedral* setting must be turned

on. These requirements ensure that the new bonds created by this style do not

create spurious 1-2, 1-3 or 1-4 interactions.

This style should be used in conjunction with a regular bond style via

:doc:`bond_style hybrid <bond_hybrid>`. Since it can be used to create

bonded interactions between particles that are further away than usual

(e.g. 1-5 or 1-6 interactions), this style may require an increase in the

communication cutoff via the :doc:`neigh_modify <neigh_modify>` command.

Restrictions

Related commands

""""""""""""""""

:doc:`bond_coeff <bond_coeff>`, :doc:`special_bonds <special_bonds>`,

:doc:`bond_coeff <bond_coeff>`, :doc:`special_bonds <special_bonds>`

**Default:** none