# fix property/atom/regiontracer/time command

## Syntax

    fix id group property/atom/regiontracer/time keyword value ...

- ID, group-ID are documented in fix command
- property/atom/regiontracer/time = style name of this fix command
- zero or more keyword/value pairs may be appended to args
- keyword = add_region or check_region_every

    add_region value = region-ID
      region-ID = ID of region to be added to list of regions where residence time is evaluated
    check_region_every value = n
      n = check every that many time-step if atom are in region

## Examples

    fix tracer all property/atom/regiontracer/time add_region tracereg

## Description

Fix property/atom/regiontracer/time computes the residence time of particles in the simulation domain and (optionally) a list of regions.

Since the look-up if a particle is in a specific region can be compuationally costly, keyword check_region_every can be used to control how often the region is checked. Every check_region_every time-steps, the lookup is performed and the residence time contribution for each lookup is dt**check_region_every*, where dt is the time-step size. However, be careful not to choose this value too large, in this case you could skip particles passing through a region.

## Restart, fix_modify, output, run start/stop, minimize info

Information about this fix is written to binary restart files .

In case no regions are specified, this fix computes a per-atom vector (the residence time of particles in the simulation domain) which can be accessed by various output commands. In case one or more optional regions are specified, this fix computes a per-atom array, where the first value for each particle is the residence time in the simulation domain and the following values are the residence time in the specified regions (in the order in which the regions are specified).

This fix computes a N-vector of residence times, where N=1+number of regions specified, which can be accessed by various output commands. The vector components are the average residence time in the fix group for the whole simulation domain (first value) and for each region (following values). The order is following the order in which the regions are specified.

## Restrictions

Currently, this feature does not support multi-sphere particles.

## Related commands

compute nparticles/tracer/region `fix property/atom/regiontracer/distance`

## Default

check_region_every = 10