# pair_style tracker command

## Syntax

``` LAMMPS
pair_style tracker fix_ID N keyword values attribute1 attribute2 ...
```

-   fix_ID = ID of associated internal fix to store data

-   N = prepare data for output every this many timesteps

-   zero or more keywords may be appended

-   keyword = *finite* or *time/min* or *type/include*

        *finite* value = none
           pair style uses atomic diameters to identify contacts
        *time/min* value = T
           T = minimum number of timesteps of interaction
        *type/include* value = list1 list2
           list1,list2 = separate lists of types (see below)

-   one or more attributes may be appended

        possible attributes = id1 id2 time/created time/broken time/total
                              r/min r/ave x y z

        id1, id2 = IDs of the 2 atoms in each pair interaction
        time/created = the timestep that the 2 atoms began interacting
        time/broken = the timestep that the 2 atoms stopped interacting
        time/total = the total number of timesteps the 2 atoms interacted
        r/min = the minimum radial distance between the 2 atoms during the interaction (distance units)
        r/ave = the average radial distance between the 2 atoms during the interaction (distance units)
        x, y, z = the center of mass position of the 2 atoms when they stopped interacting (distance units)

## Examples

``` LAMMPS
pair_style hybrid/overlay tracker myfix 1000 id1 id2 type/include 1 * type/include 2 3,4  lj/cut 2.5
pair_coeff 1 1 tracker 2.0

pair_style hybrid/overlay tracker myfix 1000 finite x y z time/min 100 granular
pair_coeff * * tracker

dump 1 all local 1000 dump.local f_myfix[1] f_myfix[2] f_myfix[3]
dump_modify 1 write_header no
```

## Description

Style *tracker* monitors information about pairwise interactions. It
does not calculate any forces on atoms. [Pair
hybrid/overlay](pair_hybrid) can be used to combine this pair style with
any other pair style, as shown in the examples above.

At each timestep, if two neighboring atoms move beyond the interaction
cutoff, pairwise data is processed and transferred to an internal fix
labeled *fix_ID*. This allows the local data to be accessed by other
LAMMPS commands. Additional filters can be applied using the *time/min*
or *type/include* keywords described below. Note that this is the
interaction cutoff defined by this pair style, not the short-range
cutoff defined by the pair style that is calculating forces on atoms.

Following any optional keyword/value arguments, a list of one or more
attributes is specified. These include the IDs of the two atoms in the
pair. The other attributes for the pair of atoms are the duration of
time they were \"interacting\" or at the point in time they started or
stopped interacting. In this context, \"interacting\" means the time
window during which the two atoms were closer than the interaction
cutoff distance. The attributes for time/\* refer to timesteps.

Data is continuously accumulated by the internal fix over intervals of
*N* timesteps. At the end of each interval, all of the saved accumulated
data is deleted to make room for new data. Individual datum may
therefore persist anywhere between *1* to *N* timesteps depending on
when they are saved. This data can be accessed using the *fix_ID* and a
[dump local](dump) command. To ensure all data is output, the dump
frequency should correspond to the same interval of *N* timesteps. A
dump frequency of an integer multiple of *N* can be used to regularly
output a sample of the accumulated data.

------------------------------------------------------------------------

The following optional keywords may be used.

If the *finite* keyword is not used, the following coefficients must be
defined for each pair of atom types via the [pair_coeff](pair_coeff)
command as in the examples above, or in the data file or restart files
read by the [read_data](read_data) or [read_restart](read_restart)
commands, or by mixing as described below:

-   cutoff (distance units)

If the *finite* keyword is used, there are no additional coefficients to
set for each pair of atom types via the [pair_coeff](pair_coeff)
command. Interaction cutoffs are instead calculated based on the
diameter of finite particles. However you must still use the
[pair_coeff](pair_coeff) for all atom types. For example the command

``` LAMMPS
pair_coeff * *
```

should be used.

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 whose
two atom types appear in *list1* and *list2*. Atom type 1 must be in
list1 and atom type 2 in list2. Or vice versa.

Each type list consists of a series of type ranges separated by commas.
Each 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). Note that the
*type/include* keyword can be specified multiple times.

## Mixing, shift, table, tail correction, restart, rRESPA info

For atom type pairs I,J and I != J, the cutoff coefficient and cutoff
distance for this pair style can be mixed. The cutoff is always mixed
via a *geometric* rule. The cutoff is mixed according to the pair_modify
mix value. The default mix value is *geometric*. See the \"pair_modify\"
command for details.

This pair style writes its information to [binary restart
files](restart), so pair_style and pair_coeff commands do not need to be
specified in an input script that reads a restart file.

The [pair_modify](pair_modify) shift, table, and tail options are not
relevant for this pair style.

The accumulated data is not written to restart files and should be
output before a restart file is written to avoid missing data.

The internal fix 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](Howto_output) page for an overview of LAMMPS output options.

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

------------------------------------------------------------------------

## Restrictions

This pair style is part of the MISC package. It is only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

This pair style is currently incompatible with granular pair styles that
extend beyond the contact (e.g. JKR and DMT).

## Related commands

## Default

none