.. index:: neighbor

neighbor command
================

Syntax
""""""

.. parsed-literal::

   neighbor skin style

* skin = extra distance beyond force cutoff (distance units)
* style = *bin* or *nsq* or *multi* or *multi/old*

Examples
""""""""

.. code-block:: LAMMPS

   neighbor 0.3 bin
   neighbor 2.0 nsq

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

This command sets parameters that affect the building of pairwise
neighbor lists.  All atom pairs within a neighbor cutoff distance
equal to the their force cutoff plus the *skin* distance are stored in
the list.  Typically, the larger the skin distance, the less often
neighbor lists need to be built, but more pairs must be checked for
possible force interactions every timestep.  The default value for
*skin* depends on the choice of units for the simulation; see the
default values below.

The *skin* distance is also used to determine how often atoms migrate
to new processors if the *check* option of the
:doc:`neigh_modify <neigh_modify>` command is set to *yes*\ .  Atoms are
migrated (communicated) to new processors on the same timestep that
neighbor lists are re-built.

The *style* value selects what algorithm is used to build the list.
The *bin* style creates the list by binning which is an operation that
scales linearly with N/P, the number of atoms per processor where N =
total number of atoms and P = number of processors.  It is almost
always faster than the *nsq* style which scales as (N/P)\^2.  For
unsolvated small molecules in a non-periodic box, the *nsq* choice can
sometimes be faster.  Either style should give the same answers.

The *multi* style is a modified binning algorithm that is useful for
systems with a wide range of cutoff distances, e.g. due to different
size particles. For granular pair styles, cutoffs are set to the
sum of the maximum atomic radii for each atom type.
For the *bin* style, the bin size is set to 1/2 of
the largest cutoff distance between any pair of atom types and a
single set of bins is defined to search over for all atom types.  This
can be inefficient if one pair of types has a very long cutoff, but
other type pairs have a much shorter cutoff. The *multi* style uses
different sized bins for collections of different sized particles, where
"size" may mean the physical size of the particle or its cutoff
distance for interacting with other particles. Different
sets of bins are then used to construct the neighbor lists as as further
described by Shire, Hanley, and Stratford :ref:`(Shire) <bytype-Shire>`.
This imposes some extra setup overhead, but the searches themselves
may be much faster. By default, each atom type defines a separate
collection of particles. For systems where two or more atom types
have the same size (either physical size or cutoff distance), the
definition of collections can be customized, which can result in less
overhead and faster performance. See the :doc:`neigh_modify <neigh_modify>`
command for how to define custom collections. Whether the collection
definition is customized or not, also see the
:doc:`comm_modify mode multi <comm_modify>` command for communication
options that further improve performance in a manner consistent with
neighbor style multi.

An alternate style, *multi/old*, sets the bin size to 1/2 of the shortest
cutoff distance and multiple sets of bins are defined to search over for
different atom types. This algorithm used to be the default *multi*
algorithm in LAMMPS but was found to be significantly slower than the new
approach. For now we are keeping the old option in case there are use cases
where multi/old outperforms the new multi style.


The :doc:`neigh_modify <neigh_modify>` command has additional options
that control how often neighbor lists are built and which pairs are
stored in the list.

When a run is finished, counts of the number of neighbors stored in
the pairwise list and the number of times neighbor lists were built
are printed to the screen and log file.  See the :doc:`Run output <Run_output>` page for details.

Restrictions
""""""""""""
 none

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

:doc:`neigh_modify <neigh_modify>`, :doc:`units <units>`,
:doc:`comm_modify <comm_modify>`

Default
"""""""

| 0.3 bin for units = lj, skin = 0.3 sigma
| 2.0 bin for units = real or metal, skin = 2.0 Angstroms
| 0.001 bin for units = si, skin = 0.001 meters = 1.0 mm
| 0.1 bin for units = cgs, skin = 0.1 cm = 1.0 mm
|

----------

.. _bytype-Shire:

**(Shire)** Shire, Hanley and Stratford, Comp Part Mech, (2020).
