lammps/doc/fix_ave_atom.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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fix ave/atom command :h3

[Syntax:]

fix ID group-ID ave/atom Nevery Nrepeat Nfreq value1 value2 ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
ave/atom = style name of this fix command :l
Nevery = calculate property every this many timesteps :l
Nrepeat = # of times to repeat the Nevery calculation before averaging :l
Nfreq = timestep frequency at which the average value is calculated :l
one or more values can be listed :l
value = x, y, z, xu, yu, zu, vx, vy, vz, fx, fy, fz, c_ID, c_ID\[i\], f_ID, f_ID\[i\], v_name :l
  x,y,z,xu,yu,zu,vx,vy,vz,fx,fy,fz = atom attribute (position, unwrapped position, velocity, force component)
  c_ID = per-atom vector calculated by a compute with ID
  c_ID\[I\] = Ith column of per-atom array calculated by a compute with ID
  f_ID = per-atom vector calculated by a fix with ID
  f_ID\[I\] = Ith column of per-atom array calculated by a fix with ID
  v_name = per-atom vector calculated by an atom-style variable with name :pre
:ule

[Examples:]

fix 1 all ave/atom 1 100 100 vx vy vz
fix 1 all ave/atom 10 20 1000 c_my_stress[1] :pre

[Description:]

Calculate one or more instantaneous per-atom quantities every few
timesteps, and average them over longer timescales.  The resulting
per-atom averages can be used by other "output
commands"_Section_howto.html#4_15 such as the "fix
ave/spatial"_fix_ave_spatial.html or "dump custom"_dump.html commands.

Each listed value is averaged independently.  The group specified with
the command means only atoms within the group have their averages
computed.  Atoms not in the group have their result(s) set to 0.0.

Each listed value can be an atom attribute (position, unwrapped
position, velocity, force component) or can be the result of a
"compute"_compute.html or "fix"_fix.html or the evaluation of an
atom-style "variable"_variable.html.  In the latter cases, the
compute, fix, or variable must produce a per-atom vector, not a global
scalar or vector or array.  If you wish to time-average global
quantities from a compute, fix, or variable, then see the "fix
ave/time"_fix_ave_time.html command.

"Computes"_compute.html that produce per-atom vectors or arrays are
those which have the word {atom} in their style name.  See the doc
pages for individual "fixes"_fix.html to determine which ones produce
per-atom vectors or arrays.  "Variables"_variable.html of style {atom}
are the only ones that can be used with this fix since they are the
only ones that produce per-atom vectors.

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The {Nevery}, {Nrepeat}, and {Nfreq} arguments specify on what
timesteps the values will be generated in order to contribute to the
average.  The final averaged quantities are generated every {Nfreq}
timesteps.  The average is over {Nrepeat} quantities, computed in the
preceding portion of the simulation every {Nevery} timesteps.  {Nfreq}
must be a multiple of {Nevery} and {Nevery} must be non-zero even if
{Nrepeat} is 1.  Also, the timesteps contributing to the average value
cannot overlap, i.e. Nfreq > (Nrepeat-1)*Nevery is required.

For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then values on
timesteps 90,92,94,96,98,100 will be used to compute the final average
on timestep 100.  Similarly for timesteps 190,192,194,196,198,200 on
timestep 200, etc.

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The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
self-explanatory.  The unwrapped values (xu,yu,zu) mean that the
coordinates are "unwrapped" by the images flags for each atom.  This
means that if the atom has passed thru a periodic boundary one or more
times, its unwrapped value is what the coordinate would be if it had
not been wrapped back into the periodic box.  Note that this means the
coordinate value may be far outside the box.  The
"dump_modify"_dump_modify.html command describes in more detail what
is meant by image flags.

If a value begins with "c_", a compute ID must follow which has been
previously defined in the input script.  If no bracketed term is
appended, the per-atom vector calculated by the compute is used.  If a
bracketed term containing an index I is appended, the Ith column of
the per-atom array calculated by the compute is used.  Users can also
write code for their own compute styles and "add them to
LAMMPS"_Section_modify.html.

If a value begins with "f_", a fix ID must follow which has been
previously defined in the input script.  If no bracketed term is
appended, the per-atom vector calculated by the fix is used.  If a
bracketed term containing an index I is appended, the Ith column of
the per-atom array calculated by the fix is used.  Note that some
fixes only produce their values on certain timesteps, which must be
compatible with {Nevery}, else an error will result.  Users can also
write code for their own fix styles and "add them to
LAMMPS"_Section_modify.html.

If a value begins with "v_", a variable name must follow which has
been previously defined in the input script as an "atom-style
variable"_variable.html Variables of style {atom} can reference
thermodynamic keywords, or invoke other computes, fixes, or variables
when they are evaluated, so this is a very general means of generating
per-atom quantities to time average.

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[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  None of the "fix_modify"_fix_modify.html options
are relevant to this fix.  No global scalar or vector quantities are
stored by this fix for access by various "output
commands"_Section_howto.html#4_15.

This fix produces a per-atom vector or array which can be accessed by
various "output commands"_Section_howto.html#4_15.  A vector is
produced if only a single quantity is averaged by this fix.  If two or
more quantities are averaged, then an array of values is produced.
The per-atom values can only be accessed on timesteps that are
multiples of {Nfreq} since that is when averaging is performed.

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[Restrictions:] none

[Related commands:]

"compute"_compute.html, "fix ave/spatial"_fix_ave_spatial.html, "dump
custom"_dump.html

[Default:] none