2007-09-28 07:28:06 +08:00
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<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>
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<HR>
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<H3>fix ave/atom command
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</H3>
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<P><B>Syntax:</B>
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</P>
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2008-01-03 03:25:15 +08:00
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<PRE>fix ID group-ID ave/atom Nevery Nrepeat Nfreq value1 value2 ...
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2007-09-28 07:28:06 +08:00
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</PRE>
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2008-01-03 03:25:15 +08:00
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>ave/atom = style name of this fix command
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<LI>Nevery = calculate property every this many timesteps
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<LI>Nrepeat = # of times to repeat the Nevery calculation before averaging
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<LI>Nfreq = timestep frequency at which the average value is calculated
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<LI>one or more values can be listed
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2009-03-18 00:11:05 +08:00
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<LI>value = x, y, z, xu, yu, zu, vx, vy, vz, fx, fy, fz, c_ID, c_ID[N], f_ID, f_ID[N], v_name
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2008-01-03 03:25:15 +08:00
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2009-03-18 00:11:05 +08:00
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<PRE> x,y,z,xu,yu,zu,vx,vy,vz,fx,fy,fz = atom attribute (position, unwrapped position, velocity, force component)
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2008-01-03 03:25:15 +08:00
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c_ID = per-atom scalar value calculated by a compute with ID
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c_ID[N] = Nth component of per-atom vector calculated by a compute with ID
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f_ID = per-atom scalar value calculated by a fix with ID
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f_ID[N] = Nth component of per-atom vector calculated by a fix with ID
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v_name = per-atom value calculated by an atom-style variable with name
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</PRE>
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2007-09-28 07:28:06 +08:00
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</UL>
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<P><B>Examples:</B>
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</P>
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2008-01-03 03:25:15 +08:00
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<PRE>fix 1 all ave/atom 1 100 100 vx vy vz
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fix 1 all ave/atom 10 20 1000 c_my_stress<B>1</B>
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2007-09-28 07:28:06 +08:00
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</PRE>
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<P><B>Description:</B>
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</P>
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2008-01-03 03:25:15 +08:00
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<P>Calculate one or more instantaneous per-atom quantities every few
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timesteps, and average them over longer timescales. The resulting
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per-atom averages can be used by other <A HREF = "Section_howto.html#4_15">output
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commands</A> such as the <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A> or <A HREF = "dump.html">dump custom</A> commands.
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</P>
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<P>Each listed value is averaged independently. The group specified with
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the command means only atoms within the group have their averages
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2008-01-04 08:56:10 +08:00
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computed. Atoms not in the group have their result(s) set to 0.0.
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2008-01-03 03:25:15 +08:00
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</P>
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2009-03-18 00:11:05 +08:00
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<P>Each listed value can be an atom attribute (position, unwrapped
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position, velocity, force component) or can be the result of a
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<A HREF = "compute.html">compute</A> or <A HREF = "fix.html">fix</A> or the evaluation of an
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atom-style <A HREF = "variable.html">variable</A>. In the latter cases, the
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compute, fix, or variable must produce a per-atom quantity, not a
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global quantity. If you wish to time-average global quantities from a
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compute, fix, or variable, then see the <A HREF = "fix_ave_time.html">fix
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ave/time</A> command.
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2008-01-03 03:25:15 +08:00
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</P>
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<P><A HREF = "compute.html">Computes</A> that produce per-atom quantities are those
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2008-01-04 07:28:08 +08:00
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which have the word <I>atom</I> in their style name. See the doc pages for
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2008-02-20 05:47:37 +08:00
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individual <A HREF = "fix.html">fixes</A> to determine which ones produce per-atom
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2008-01-04 07:28:08 +08:00
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quantities. <A HREF = "variable.html">Variables</A> of style <I>atom</I> are the only
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ones that can be used with this fix since all other styles of variable
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produce global quantities.
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2008-01-03 03:25:15 +08:00
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</P>
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<HR>
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<P>The <I>Nevery</I>, <I>Nrepeat</I>, and <I>Nfreq</I> arguments specify on what
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timesteps the values will be generated in order to contribute to the
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average. The final averaged quantities are generated every <I>Nfreq</I>
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timesteps. The average is over <I>Nrepeat</I> quantities, computed in the
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2008-02-20 05:47:37 +08:00
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preceding portion of the simulation every <I>Nevery</I> timesteps. <I>Nfreq</I>
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must be a multiple of <I>Nevery</I> and <I>Nevery</I> must be non-zero even if
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2008-05-27 22:03:44 +08:00
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<I>Nrepeat</I> is 1. Also, the timesteps contributing to the average value
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cannot overlap, i.e. Nfreq > (Nrepeat-1)*Nevery is required.
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2008-01-03 03:25:15 +08:00
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</P>
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<P>For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then values on
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timesteps 90,92,94,96,98,100 will be used to compute the final average
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2008-02-20 05:47:37 +08:00
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on timestep 100. Similarly for timesteps 190,192,194,196,198,200 on
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2008-01-03 03:25:15 +08:00
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timestep 200, etc.
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</P>
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<HR>
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<P>The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
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2009-03-18 00:11:05 +08:00
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self-explanatory. The unwrapped values (xu,yu,zu) mean that the
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coordinates are "unwrapped" by the images flags for each atom. This
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means that if the atom has passed thru a periodic boundary one or more
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times, its unwrapped value is what the coordinate would be if it had
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not been wrapped back into the periodic box. Note that this means the
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coordinate value may be far outside the box. The
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<A HREF = "dump_modify.html">dump_modify</A> command describes in more detail what
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is meant by image flags.
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2008-01-03 03:25:15 +08:00
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</P>
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<P>If a value begins with "c_", a compute ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the per-atom scalar calculated by the compute is used. If a
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bracketed term is appended, the Nth vector per-atom value calculated
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by the compute is used. Users can also write code for their own
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compute styles and <A HREF = "Section_modify.html">add them to LAMMPS</A>.
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</P>
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<P>If a value begins with "f_", a fix ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the per-atom scalar calculated by the fix is used. If a
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bracketed term is appended, the Nth vector per-atom value calculated
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by the fix is used. Note that some fixes only produce their values on
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certain timesteps, which must be compatible with <I>Nevery</I>, else an
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error will results. Users can also write code for their own fix
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styles and <A HREF = "Section_modify.html">add them to LAMMPS</A>.
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</P>
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<P>If a value begins with "v_", a variable name must follow which has
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been previously defined in the input script. Variables of style
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<I>atom</I> can reference thermodynamic keywords, or invoke other computes,
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fixes, or variables when they are evaluated, so this is a very general
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means of generating per-atom quantities to time average.
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2007-09-28 07:28:06 +08:00
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</P>
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2009-11-18 23:22:46 +08:00
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<HR>
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2007-10-11 06:28:11 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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2007-09-28 07:28:06 +08:00
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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2007-10-11 06:28:11 +08:00
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are relevant to this fix. No global scalar or vector quantities are
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stored by this fix for access by various <A HREF = "Section_howto.html#4_15">output
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commands</A>.
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</P>
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2008-01-03 03:25:15 +08:00
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<P>This fix produces a per-atom scalar or vector which can be accessed by
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various <A HREF = "Section_howto.html#4_15">output commands</A>. A scalar is
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produced if only a single quantity is averaged by this fix. If two or
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more quantities are averaged, then a vector of values is produced.
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The per-atom values can only be accessed on timesteps that are
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multiples of <I>Nfreq</I> since that is when averaging is performed.
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2007-10-11 06:28:11 +08:00
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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2007-09-28 07:28:06 +08:00
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "compute.html">compute</A>, <A HREF = "fix_ave_spatial.html">fix ave/spatial</A>, <A HREF = "dump.html">dump
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custom</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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