# fix property/atom/tracer command

## Syntax

    fix id group property/atom/tracer region_mark region-ID mark_step s keyword value ...

- ID, group-ID are documented in fix command
- property/atom/tracer = style name of this fix command
- region_mark = obligatory keyword
- region-ID = ID of region atoms must be in to be marked
- mark_step = obligatory keyword
- s = step when atoms are marked (or started to be marked, depending on marker_style)
- zero or more keyword/value pairs may be appended to args
- keyword = marker_style or check_mark_every

    marker_style value = dirac or heaviside
      dirac = use a dirac impulse at time step s to mark the particles
      heaviside = use a dirac impulse staring at time step s to mark the particles
    check_mark_every value = n
      n = check every that many time-step if atom are in region to be marked

## Examples

    fix tracer all property/atom/tracer region_mark mark mark_step 10000 marker_style dirac check_mark_every 10

## Description

Fix property/atom/tracer marks particles using either a Dirac delta impulse (default) or a Heaviside impulse, as specified by the marker_style keyword. Particles are marked if they are inside the region specified by the region_mark keyword. Using the Dirac impulse means that all the particles which are in the region at the time-step specified by the mark_step keyword are marked. Using the Heaviside impulse means that all the particles which pass by the specified region after the specified time-step are marked.

Keyword check_mark_every can be used to control how often the region is checked. Typically, this is useful when the heaviside option is used, because you may not want to check each particle each time-step. However, be careful not to choose this value too large, in this case you could skip particles passing through the region.

It is useful to combine this command with a compute nparticles/tracer/region command to compute residence time distributions.

Warning

Using compute nparticles/tracer/region can change the tracer value (keyword reset_marker.)

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

Information about this fix is written to binary restart files .

This fix computes a per-atom scalar (the marker) which can be accessed by various output commands. This scalar is equal to 1 if a particle has been marked and 0 otherwise. This fix also computes a global scalar indicating how many particles were marked since the last time the global scalar was computed. This scalar can also be accessed by various output commands.

## Restrictions

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

## Related commands

compute nparticles/tracer/region

## Default

marker_style = dirac, check_mark_every = 10