LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

neighbor command

Syntax:

neighbor skin style 

Examples:

neighbor 0.3 bin
neighbor 2.0 nsq 

Description:

This command sets parameters that affect the building of the pairwise neighbor list. All atom pairs within a 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 below).

For simulations without pairwise interactions (see "pair_style none" command), and just bonded interactions, you still need to set the neighbor skin distance. Pairwise neighbor lists will not be formed but the skin distance also determines which atoms are communicated from nearby processors, so it should be slightly large enough that all bond, angle, etc atoms are acquired.

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 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. For style multi the bin size is set to 1/2 of the shortest cutoff distance and multiple sets of bins are defined to search over for different atom types. This imposes some extra setup overhead, but the searches themselves may be much faster for the short-cutoff cases.

The 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 this section for details.

Restrictions: none

Related commands:

neigh_modify, units

Default:

0.3 bin      for lj units (0.3 sigma)
2.0 bin      for real or metal units (2.0 Angstroms)