.. index:: create_bonds

create_bonds command
====================

Syntax
""""""

.. parsed-literal::

   create_bonds group-ID group2-ID btype rmin rmax

* group-ID = ID of first group
* group2-ID = ID of second group, bonds will be between atoms in the 2 groups
* btype = bond type of created bonds
* rmin = minimum distance between pair of atoms to bond together
* rmax = minimum distance between pair of atoms to bond together

Examples
""""""""

.. parsed-literal::

   create_bonds all all 1 1.0 1.2
   create_bonds surf solvent 3 2.0 2.4

Description
"""""""""""

Create bonds between pairs of atoms that meet specified distance
criteria.  The bond interactions can then be computed during a
simulation by the bond potential defined by the
:doc:`bond_style <bond_style>` and :doc:`bond_coeff <bond_coeff>`
commands.  This command is useful for adding bonds to a system,
e.g. between nearest neighbors in a lattice of atoms, without having
to enumerate all the bonds in the data file read by the
:doc:`read_data <read_data>` command.

Note that the flexibility of this command is limited.  It can be used
several times to create different types of bond at different
distances.  But it cannot typically create all the bonds that would
normally be defined in a complex system of molecules.  Also note that
this command does not add any 3-body or 4-body interactions which,
depending on your model, may be induced by added bonds,
e.g. :doc:`angle <angle_style>`, :doc:`dihedral <dihedral_style>`, or
:doc:`improper <improper_style>` interactions.

All created bonds will be between pairs of atoms I,J where I is in one
of the two specified groups, and J is in the other.  The two groups
can be the same, e.g. group "all".  The created bonds will be of bond
type *btype*\ , where *btype* must be a value between 1 and the number
of bond types defined.  This maximum value is set by the "bond types"
field in the header of the data file read by the
:doc:`read_data <read_data>` command, or via the optional "bond/types"
argument of the :doc:`create_box <create_box>` command.

For a bond to be created, an I,J pair of atoms must be a distance D
apart such that *rmin* <= D <= *rmax*\ .

The following settings must have been made in an input
script before this command is used:

* special_bonds weight for 1-2 interactions must be 0.0
* a :doc:`pair_style <pair_style>` must be defined
* no :doc:`kspace_style <kspace_style>` defined
* minimum :doc:`pair_style <pair_style>` cutoff + :doc:`neighbor <neighbor>` skin >= *rmax*

These settings are required so that a neighbor list can be created to
search for nearby atoms.  Pairs of atoms that are already bonded
cannot appear in the neighbor list, to avoid creation of duplicate
bonds.  The neighbor list for all atom type pairs must also extend to
a distance that encompasses the *rmax* for new bonds to create.

An additional requirement is that your system must be ready to perform
a simulation.  This means, for example, that all
:doc:`pair_style <pair_style>` coefficients be set via the
:doc:`pair_coeff <pair_coeff>` command.  A :doc:`bond_style <bond_style>`
command and all bond coefficients must also be set, even if no bonds
exist before this command is invoked.  This is because the building of
neighbor list requires initialization and setup of a simulation,
similar to what a :doc:`run <run>` command would require.

Note that you can change any of these settings after this command
executes, e.g. if you wish to use long-range Coulombic interactions
via the :doc:`kspace_style <kspace_style>` command for your subsequent
simulation.

.. note::

   If the system has no bonds to begin with, or if more bonds per
   atom are being added than currently exist, then you must insure that
   the number of bond types and the maximum number of bonds per atom are
   set to large enough values.  Otherwise an error may occur when too
   many bonds are added to an atom.  If the :doc:`read_data <read_data>`
   command is used to define the system, these 2 parameters can be set
   via the "bond types" and "extra bond per atom" fields in the header
   section of the data file.  If the :doc:`create_box <create_box>` command
   is used to define the system, these 2 parameters can be set via its
   optional "bond/types" and "extra/bond/per/atom" arguments.  See the
   doc pages for the 2 commands for details.

Restrictions
""""""""""""


This command cannot be used with molecular systems defined using
molecule template files via the :doc:`molecule <molecule>` and
:doc:`atom_style template <atom_style>` commands.

Related commands
""""""""""""""""

:doc:`create_atoms <create_atoms>`, :doc:`delete_bonds <delete_bonds>`

**Default:** none


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm