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### compute nparticles/tracer/region command

Syntax:

    compute ID group-ID nparticles/tracer/region

- ID, group-ID are documented in compute command
- nparticles/tracer/region = style name of this compute command
- region_count = obligatory keyword
- region-ID = ID of region atoms must be in to be counted
- tracer = obligatory keyword
- tracer-ID = ID of a fix of type fix property/atom/tracer
- zero or more keyword/value pairs may be appended to args
- keyword = periodic or check_mark_every

        periodic value = dim image
          dim = x or y or z
          image = image that a particle has to be in to be counted (any integer number or all)
        reset_marker value = yes or no
          yes = un-mark particles after counting them
          no = do not un-mark particles after counting them

  Examples:

      compute nparticles all nparticles/tracer/region region_count count tracer tr periodic z -1

  Description:

  Define a computation that calculates the number and mass of marked and un-marked particles that are in the region speficied via the region_count keyword. Particles have to be in the group "group-ID" to be counted.

  Note that only particles marked by a fix property/atom/tracer or fix property/atom/tracer/stream command are counted - therefore, a valid ID of such a fix has to be provided via the tracer keyword.

  The reset_marker keyword controls if particles are un-marked (default) after they have been counted once by this command.

  IMPORTANT NOTE: If multiple compute nparticles/tracer/region commands are operating on the same fix property/atom/tracer commands, and the first compute resets the marker value, the second compute will not count them.

  With the periodic keyword, you can restrict counting/unmarking to particles which are in a specified image in a periodic simulation. For example, using

      periodic z +2

  means that particles are only counted if they are in z-image #2. By default, all particles are counted/unmarked regardless in which periodic image they are.

  IMPORTANT NOTE: Currently, this command only supports one periodic boundary restriction via the periodic keyword. If keyword periodic is used multiple times, the last setting will be applied.

  Output info:

  This this compute calculates a global vector containing the following information (the number in brackets corresponds to the vector id):

- (1) total number of (marked + un-marked) particles in region
- (2) number of marked particles in region
- (3) total mass of (marked + un-marked) particles in region
- (4) mass of marked particles in region

  See this section for an overview of LAMMPS output options.

  Restrictions:

  Currently, only one periodic restriction via the periodic keyword can be used.

  Related commands:

  fix property/atom/tracer

  Default: reset_marker = yes, periodic is off per default