fix pair/tracker command


fix ID group-ID pair/tracker N attribute1 attribute2 ... keyword values ...
  • ID, group-ID are documented in fix command

  • pair/tracker = style name of this fix command

  • N = prepare data for output every this many timesteps

  • one or more attributes may be appended

    possible attributes = id1 id2 time/created time/broken time/total
                          rmin rave x y z
    id1, id2 = IDs of the 2 atoms in each pair interaction
    time/created = the time that the 2 atoms began interacting
    time/broken = the time that the 2 atoms stopped interacting
    time/total = the total time the 2 atoms interacted
    r/min = the minimum radial distance between the 2 atoms during the interaction
    r/ave = the average radial distance between the 2 atoms during the interaction
    x, y, z = the center of mass position of the 2 atoms when they stopped interacting
  • zero or more keyword/value pairs may be appended

  • keyword = time/min or type/include

    time/min value = T
      T = minimum interaction time
    type/include value = arg1 arg2
      arg = separate lists of types (see below)


fix 1 all pair/tracker 1000 id1 id2 time/min 100
fix 1 all pair/tracker 1000 time/created time/broken type/include 1 * type/include 2 3,4


Tracks properties of pairwise interactions between two atoms and records data whenever the atoms move beyond the interaction cutoff. Must be used in conjunction with pair tracker. Data is accumulated over a span of N timesteps before being deleted. The number of datums generated, aggregated across all processors, equals the number of broken interactions. Interactions are only included if both atoms are included in the specified fix group. Additional filters can be applied using the time/min or type/include keywords described below.


For extremely long-lived interactions, the calculation of r/ave may not be correct due to double overflow.

The time/min keyword sets a minimum amount of time that an interaction must persist to be included. This setting can be used to censor short-lived interactions. The type/include keyword filters interactions based on the types of the two atoms. Data is only saved for interactions between atoms with types in the two lists. Each list consists of a series of type ranges separated by commas. The range can be specified as a single numeric value, or a wildcard asterisk can be used to specify a range of values. This takes the form “*” or “*n” or “n*” or “m*n”. For example, if M = the number of atom types, then an asterisk with no numeric values means all types from 1 to M. A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to M (inclusive). A middle asterisk means all types from m to n (inclusive). Multiple type/include keywords may be added.

Restart, fix_modify, run start/stop, minimize info

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No parameter of this fix can be used with the start/stop keywords of the run command.

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 recorded, lost interactions. 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 the Howto output page for an overview of LAMMPS output options.

The vector or array values will be doubles that correspond to the specified attribute.


Must be used in conjunction with pair style tracker.

This fix is part of the MISC package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.