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### compute pair/local command

Syntax:

    compute ID group-ID pair/local value1 value2 ... keyword args ...

- ID, group-ID are documented in compute command
- pair/local = style name of this compute command
- one or more values may be appended
- value = dist or eng or force or fx or fy or fz or pN

        dist = pairwise distance
        eng = pairwise energy
        force = pairwise force
        fx,fy,fz = components of pairwise force
        pN = pair style specific quantities for allowed N values

- zero or more keyword/arg pairs may be appended
- keyword = cutoff

        cutoff arg = type or radius

Examples:

    compute 1 all pair/local eng
    compute 1 all pair/local dist eng force
    compute 1 all pair/local dist eng fx fy fz
    compute 1 all pair/local dist fx fy fz p1 p2 p3

Description:

Define a computation that calculates properties of individual pairwise interactions. The number of datums generated, aggregated across all processors, equals the number of pairwise interactions in the system.

The local data stored by this command is generated by looping over the pairwise neighbor list. Info about an individual pairwise interaction will only be included if both atoms in the pair are in the specified compute group, and if the current pairwise distance is less than the force cutoff distance for that interaction, as defined by the pair_style and pair_coeff commands.

The value dist is the distance bewteen the pair of atoms.

The value eng is the interaction energy for the pair of atoms.

The value force is the force acting between the pair of atoms, which is positive for a repulsive force and negative for an attractive force. The values fx, fy, and fz are the xyz components of force on atom I.

A pair style may define additional pairwise quantities which can be accessed as p1 to pN, where N is defined by the pair style. Most pair styles do not define any additional quantities, so N = 0. An example of ones that do are the granular pair styles which calculate the tangential force between two particles and return its components and magnitude acting on atom I for N = 1,2,3,4. See individual pair styles for detils.

The value dist will be in distance units. The value eng will be in energy units. The values force, fx, fy, and fz will be in force units. The values pN will be in whatever units the pair style defines.

The optional cutoff keyword determines how the force cutoff distance for an interaction is determined. For the default setting of type, the pairwise cutoff defined by the pair_style command for the types of the two atoms is used. For the radius setting, the sum of the radii of the two particles is used as a cutoff. For example, this is appropriate for granular particles which only interact when they are overlapping, as computed by granular pair styles.

Note that as atoms migrate from processor to processor, there will be no consistent ordering of the entries within the local vector or array from one timestep to the next. The only consistency that is guaranteed is that the ordering on a particular timestep will be the same for local vectors or arrays generated by other compute commands. For example, pair output from the compute property/local command can be combined with data from this command and output by the dump local command in a consistent way.

Here is an example of how to do this:

    compute 1 all property/local patom1 patom2
    compute 2 all pair/local dist eng force
    dump 1 all local 1000 tmp.dump index c_1[1] c_1[2] c_2[1] c_2[2] c_2[3]

NOTE: For pairs, if two atoms I,J are involved in 1-2, 1-3, 1-4 interactions within the molecular topology, their pairwise interaction may be turned off, and thus they may not appear in the neighbor list, and will not be part of the local data created by this command. More specifically, this will be true of I,J pairs with a weighting factor of 0.0; pairs with a non-zero weighting factor are included. The weighting factors for 1-2, 1-3, and 1-4 pairwise interactions are set by the special_bonds command. An exception is if long-range Coulombics are being computed via the kspace_style command, then atom pairs with weighting factors of zero are still included in the neighbor list, so that a portion of the long-range interaction contribution can be computed in the pair style. Hence in that case, those atom pairs will be part of the local data created by this command.

Output info:

This compute calculates a local vector or local array depending on the number of keywords. The length of the vector or number of rows in the array is the number of pairs. If a single keyword is specified, a local vector is produced. If two or more keywords are specified, a local array is produced where the number of columns = the number of keywords. The vector or array can be accessed by any command that uses local values from a compute as input. See this section for an overview of LAMMPS output options.

The output for dist will be in distance units. The output for eng will be in energy units. The output for force, fx, fy, and fz will be in force units. The outpur for pN will be in whatever units the pair style defines.

Restrictions: none

Related commands:

dump local, compute property/local

Default:

The keyword default is cutoff = type.