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fix_ave_atom.html
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>fix ave/atom command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID ave/atom Nevery Nrepeat Nfreq value1 value2 ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>ave/atom = style name of this fix command
<LI>Nevery = use input values every this many timesteps
<LI>Nrepeat = # of times to use input values for calculating averages
<LI>Nfreq = calculate averages every this many timesteps
one or more input values can be listed
<LI>value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[i], f_ID, f_ID[i], v_name
<PRE> x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (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>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all ave/atom 1 100 100 vx vy vz
fix 1 all ave/atom 10 20 1000 c_my_stress[1]
</PRE>
<P><B>Description:</B>
</P>
<P>Use one or more per-atom vectors as inputs every few timesteps, and
average them atom by atom over longer timescales. The resulting
per-atom averages can be used by other <A HREF = "Section_howto.html#howto_15">output
commands</A> such as the <A HREF = "fix_ave_spatial.html">fix
ave/spatial</A> or <A HREF = "dump.html">dump custom</A> commands.
</P>
<P>The group specified with the command means only atoms within the group
have their averages computed. Results are set to 0.0 for atoms not in
the group.
</P>
<P>Each input value can be an atom attribute (position, velocity, force
component) or can be the result of a <A HREF = "compute.html">compute</A> or
<A HREF = "fix.html">fix</A> or the evaluation of an atom-style
<A HREF = "variable.html">variable</A>. In the latter cases, the compute, fix, or
variable must produce a per-atom vector, not a global quantity or
local quantity. If you wish to time-average global quantities from a
compute, fix, or variable, then see the <A HREF = "fix_ave_time.html">fix
ave/time</A> command.
</P>
<P><A HREF = "compute.html">Computes</A> that produce per-atom vectors or arrays are
those which have the word <I>atom</I> in their style name. See the doc
pages for individual <A HREF = "fix.html">fixes</A> to determine which ones produce
per-atom vectors or arrays. <A HREF = "variable.html">Variables</A> of style <I>atom</I>
are the only ones that can be used with this fix since they produce
per-atom vectors.
</P>
<P>Each per-atom value of each input vector is averaged independently.
</P>
<HR>
<P>The <I>Nevery</I>, <I>Nrepeat</I>, and <I>Nfreq</I> arguments specify on what
timesteps the input values will be used in order to contribute to the
average. The final averaged quantities are generated on timesteps
that are a multiple of <I>Nfreq</I>. The average is over <I>Nrepeat</I>
quantities, computed in the preceding portion of the simulation every
<I>Nevery</I> timesteps. <I>Nfreq</I> must be a multiple of <I>Nevery</I> and
<I>Nevery</I> must be non-zero even if <I>Nrepeat</I> is 1. Also, the timesteps
contributing to the average value cannot overlap, i.e. Nfreq >
(Nrepeat-1)*Nevery is required.
</P>
<P>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.
</P>
<HR>
<P>The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
self-explanatory. Note that other atom attributes can be used as
inputs to this fix by using the <A HREF = "compute_property_atom.html">compute
property/atom</A> command and then specifying
an input value from that compute.
</P>
<P>IMPORTANT NOTE: The x,y,z attributes are values that are re-wrapped
inside the periodic box whenever an atom crosses a periodic boundary.
Thus if you time average an atom that spends half its time on either
side of the periodic box, you will get a value in the middle of the
box. If this is not what you want, consider averaging unwrapped
coordinates, which can be provided by the <A HREF = "compute_property_atom.html">compute
property/atom</A> command via its xu,yu,zu
attributes.
</P>
<P>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 <A HREF = "Section_modify.html">add them to
LAMMPS</A>.
</P>
<P>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 <I>Nevery</I>, else an error will result. Users can also
write code for their own fix styles and <A HREF = "Section_modify.html">add them to
LAMMPS</A>.
</P>
<P>If a value begins with "v_", a variable name must follow which has
been previously defined in the input script as an <A HREF = "variable.html">atom-style
variable</A> Variables of style <I>atom</I> 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.
</P>
<HR>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix. No global scalar or vector quantities are
stored by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
commands</A>.
</P>
<P>This fix produces a per-atom vector or array which can be accessed by
various <A HREF = "Section_howto.html#howto_15">output commands</A>. 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 <I>Nfreq</I> since that is when averaging is performed.
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
minimization</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute.html">compute</A>, <A HREF = "fix_ave_histo.html">fix ave/histo</A>, <A HREF = "fix_ave_spatial.html">fix
ave/spatial</A>, <A HREF = "fix_ave_time.html">fix ave/time</A>,
<A HREF = "variable.html">variable</A>,
</P>
<P><B>Default:</B> none
</P>
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