LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

### compute property/local command

Syntax:

    compute ID group-ID property/local input1 input2 ...

- ID, group-ID are documented in compute command
- property/local = style name of this compute command
- input = one or more attributes

        possible attributes = natom1 natom2
      		        patom1 patom2
                              batom1 batom2 btype
                              aatom1 aatom2 aatom3 atype
                              datom1 datom2 datom3 dtype
                              iatom1 iatom2 iatom3 itype

           natom1, natom2 = IDs of 2 atoms in each pair (within neighbor cutoff)
           patom1, patom2 = IDs of 2 atoms in each pair (within force cutoff)
           batom1, batom2 = IDs of 2 atoms in each bond
           btype = bond type of each bond
           aatom1, aatom2, aatom3 = IDs of 3 atoms in each angle
           atype = angle type of each angle
           datom1, datom2, datom3, datom4 = IDs of 4 atoms in each dihedral
           dtype = dihedral type of each dihedral
           iatom1, iatom2, iatom3, iatom4 = IDs of 4 atoms in each improper
           itype = improper type of each improper

Examples:

    compute 1 all property/local btype batom1 batom2
    compute 1 all property/local atype aatom2

Description:

Define a computation that stores the specified attributes as local data so it can be accessed by other output commands. If the input attributes refer to bond information, then the number of datums generated, aggregated across all processors, equals the number of bonds in the system. Ditto for pairs, angles, etc.

If multiple input attributes are specified then they must all generate the same amount of information, so that the resulting local array has the same number of rows for each column. This means that only bond attributes can be specified together, or angle attributes, etc. Bond and angle attributes can not be mixed in the same compute property/local command.

If the inputs are pair attributes, the local data 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. For natom1 and natom2, all atom pairs in the neighbor list are considered (out to the neighbor cutoff = force cutoff + neighbor skin). For patom1 and patom2, the distance between the atoms must be less than the force cutoff distance for that pair to be included, as defined by the pair_style and pair_coeff commands.

If the inputs are bond, angle, etc attributes, the local data is generated by looping over all the atoms owned on a processor and extracting bond, angle, etc info. For bonds, info about an individual bond will only be included if both atoms in the bond are in the specified compute group. Likewise for angles, dihedrals, etc.

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, output from the compute bond/local command can be combined with bond atom indices from this command and output by the dump local command in a consistent way.

The natom1 and natom2, or patom1 and patom2 attributes refer to the atom IDs of the 2 atoms in each pairwise interaction computed by the pair_style command.

IMPORTANT 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 may 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.

The batom1 and batom2 attributes refer to the atom IDs of the 2 atoms in each bond. The btype attribute refers to the type of the bond, from 1 to Nbtypes = # of bond types. The number of bond types is defined in the data file read by the read_data command. The attributes that start with "a", "d", "i", refer to similar values for angles, dihedrals, and impropers.

Output info:

This compute calculates a local vector or local array depending on the number of input values. The length of the vector or number of rows in the array is the number of bonds, angles, etc. If a single input is specified, a local vector is produced. If two or more inputs are specified, a local array is produced where the number of columns = the number of inputs. 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 vector or array values will be integers that correspond to the specified attribute.

Restrictions: none

Related commands:

dump local, compute reduce

Default: none