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309 lines
15 KiB
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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/time command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID ave/time Nevery Nrepeat Nfreq value1 value2 ... keyword args ...
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</PRE>
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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/time = style name of this fix command
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<LI>Nevery = use input values every this many timesteps
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<LI>Nrepeat = # of times to use input values for calculating averages
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<LI>Nfreq = calculate averages every this many timesteps
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one or more input values can be listed
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<LI>value = c_ID, c_ID[N], f_ID, f_ID[N], v_name
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<PRE> c_ID = global scalar or vector calculated by a compute with ID
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c_ID[I] = Ith component of global vector or Ith column of global array calculated by a compute with ID
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f_ID = global scalar or vector calculated by a fix with ID
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f_ID[I] = Ith component of global vector or Ith column of global array calculated by a fix with ID
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v_name = global value calculated by an equal-style variable with name
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</PRE>
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<LI>zero or more keyword/arg pairs may be appended
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<LI>keyword = <I>mode</I> or <I>file</I> or <I>ave</I> or <I>start</I> or <I>off</I> or <I>title1</I> or <I>title2</I> or <I>title3</I>
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<PRE> <I>mode</I> arg = <I>scalar</I> or <I>vector</I>
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scalar = all input values are global scalars
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vector = all input values are global vectors
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<I>file</I> arg = filename
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filename = name of file to output time averages to
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<I>ave</I> args = <I>one</I> or <I>running</I> or <I>window M</I>
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one = output a new average value every Nfreq steps
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running = output cumulative average of all previous Nfreq steps
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window M = output average of M most recent Nfreq steps
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<I>start</I> args = Nstart
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Nstart = start averaging on this timestep
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<I>off</I> arg = M = do not average this value
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M = value # from 1 to Nvalues
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<I>title1</I> arg = string
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string = text to print as 1st line of output file
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<I>title2</I> arg = string
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string = text to print as 2nd line of output file
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<I>title3</I> arg = string
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string = text to print as 3rd line of output file, only for vector mode
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all ave/time 100 5 1000 c_myTemp c_thermo_temp file temp.profile
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fix 1 all ave/time 100 5 1000 c_thermo_press[2] ave window 20 &
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title1 "My output values"
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fix 1 all ave/time 1 100 1000 f_indent f_indent[1] file temp.indent off 1
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Use one or more global values as inputs every few timesteps, and
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average them over longer timescales. The resulting averages can be
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used by other <A HREF = "Section_howto.html#4_15">output commands</A> such as
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<A HREF = "thermo_style.html">thermo_style custom</A>, and can also be written to a
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file. Note that if no time averaging is done, this command can be
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used as a convenient way to simply output one or more global values to
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a file.
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</P>
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<P>The group specified with this command is ignored. However, note that
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calculations may be performed by computes and fixes which store their
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own "group" definitions.
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</P>
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<P>Each input value can be the result of a <A HREF = "compute.html">compute</A> or
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<A HREF = "fix.html">fix</A> or the evaluation of an equal-style
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<A HREF = "variable.html">variable</A>. In each case, the compute, fix, or variable
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must produce a global quantity, not a per-atom or local quantity. If
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you wish to spatial- or time-average or histogram per-atom quantities
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from a compute, fix, or variable, then see the <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A>, <A HREF = "fix_ave_atom.html">fix ave/atom</A>,
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or <A HREF = "fix_ave_histo.html">fix ave/histo</A> commands. If you wish to sum a
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per-atom quantity into a single global quantity, see the <A HREF = "compute_reduce.html">compute
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reduce</A> command.
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</P>
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<P><A HREF = "compute.html">Computes</A> that produce global quantities are those which
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do not have the word <I>atom</I> in their style name. Only a few
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<A HREF = "fix.html">fixes</A> produce global quantities. See the doc pages for
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individual fixes for info on which ones produce such values.
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<A HREF = "variable.html">Variables</A> of style <I>equal</I> are the only ones that can
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be used with this fix. Variables of style <I>atom</I> cannot be used,
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since they produce per-atom values.
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</P>
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<P>The listed values must either be all scalars or all vectors, depending
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on the setting of the <I>mode</I> option. In both cases, the averaging is
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performed independently on each input value. I.e. each input scalar
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or the elements of each input vector is averaged independently.
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</P>
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<P>If <I>mode</I> = vector, then all the input vectors must be the same
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length.
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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 input values will be used 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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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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<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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</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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on timestep 100. Similarly for timesteps 190,192,194,196,198,200 on
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timestep 200, etc. If Nrepeat=1 and Nfreq = 100, then no time
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averaging is done; values are simply generated on timesteps
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100,200,etc.
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</P>
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<HR>
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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 <I>mode</I> = scalar, then if
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no bracketed term is appended, the global scalar calculated by the
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compute is used. If a bracketed term is appended, the Ith element of
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the global vector calculated by the compute is used. If <I>mode</I> =
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vector, then if no bracketed term is appended, the global vector
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calculated by the compute is used. If a bracketed term is appended,
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the Ith column of the global array calculated by the compute is used.
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</P>
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<P>Note that there is a <A HREF = "compute_reduce.html">compute reduce</A> command
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which can sum per-atom quantities into a global scalar or vector which
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can thus be accessed by fix ave/time. Or it can be a compute defined
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not in your input script, but by <A HREF = "thermo_style.html">thermodynamic
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output</A> or other fixes such as <A HREF = "fix_nvt.html">fix
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nvt</A> or <A HREF = "fix_temp_rescale.html">fix temp/rescale</A>. See
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the doc pages for these commands which give the IDs of these computes.
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Users can also write code for their own compute styles and <A HREF = "Section_modify.html">add them
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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 <I>mode</I> = scalar, then if
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no bracketed term is appended, the global scalar calculated by the fix
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is used. If a bracketed term is appended, the Ith element of the
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global vector calculated by the fix is used. If <I>mode</I> = vector, then
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if no bracketed term is appended, the global vector calculated by the
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fix is used. If a bracketed term is appended, the Ith column of the
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global array calculated by the fix is used.
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</P>
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<P>Note that some fixes only produce their values on certain timesteps,
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which must be compatible with <I>Nevery</I>, else an error will result.
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Users can also write code for their own fix styles and <A HREF = "Section_modify.html">add them to
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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 can only be
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used as input for <I>mode</I> = scalar. Only equal-style variables can be
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referenced. See the <A HREF = "variable.html">variable</A> command for details.
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Note that variables of style <I>equal</I> define a formula which can
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reference individual atom properties or thermodynamic keywords, or
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they can invoke other computes, fixes, or variables when they are
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evaluated, so this is a very general means of specifying quantities to
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time average.
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</P>
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<HR>
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<P>Additional optional keywords also affect the operation of this fix.
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</P>
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<P>If the <I>mode</I> keyword is set to <I>scalar</I>, then all input values must
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be global scalars, or elements of global vectors. If the <I>mode</I>
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keyword is set to <I>vector</I>, then all input values must be global
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vectors, or columns of global arrays.
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</P>
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<P>The <I>ave</I> keyword determines how the values produced every <I>Nfreq</I>
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steps are averaged with values produced on previous steps that were
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multiples of <I>Nfreq</I>, before they are accessed by another output
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command or written to a file.
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</P>
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<P>If the <I>ave</I> setting is <I>one</I>, then the values produced on timesteps
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that are multiples of <I>Nfreq</I> are independent of each other; they are
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output as-is without further averaging.
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</P>
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<P>If the <I>ave</I> setting is <I>running</I>, then the values produced on
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timesteps that are multiples of <I>Nfreq</I> are summed and averaged in a
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cumulative sense before being output. Each output value is thus the
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average of the value produced on that timestep with all preceding
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values. This running average begins when the fix is defined; it can
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only be restarted by deleting the fix via the <A HREF = "unfix.html">unfix</A>
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command, or by re-defining the fix by re-specifying it.
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</P>
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<P>If the <I>ave</I> setting is <I>window</I>, then the values produced on
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timesteps that are multiples of <I>Nfreq</I> are summed and averaged within
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a moving "window" of time, so that the last M values are used to
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produce the output. E.g. if M = 3 and Nfreq = 1000, then the output
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on step 10000 will be the average of the individual values on steps
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8000,9000,10000. Outputs on early steps will average over less than M
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values if they are not available.
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</P>
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<P>The <I>start</I> keyword specifies what timestep averaging will begin on.
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The default is step 0. Often input values can be 0.0 at time 0, so
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setting <I>start</I> to a larger value can avoid including a 0.0 in a
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running or windowed average.
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</P>
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<P>The <I>off</I> keyword can be used to flag any of the input values. If a
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value is flagged, it will not be time averaged. Instead the most
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recent input value will always be stored and output. This is useful
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if one of more of the inputs produced by a compute or fix or variable
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are effectively constant or are simply current values. E.g. they are
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being written to a file with other time-averaged values for purposes
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of creating well-formatted output.
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</P>
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<P>The <I>file</I> keyword allows a filename to be specified. Every <I>Nfreq</I>
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steps, one quantity or vector of quantities is written to the file for
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each input value specified in the fix ave/time command. For <I>mode</I> =
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scalar, this means a single line is written each time output is
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performed. Thus the file ends up to be a series of lines, i.e. one
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column of numbers for each input value. For <I>mode</I> = vector, an array
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of numbers is written each time output is performed. The number of
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rows is the length of the input vectors, and the number of columns is
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the number of values. Thus the file ends up to be a series of these
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array sections.
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</P>
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<P>The <I>title1</I> and <I>title2</I> and <I>title3</I> keywords allow specification of
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the strings that will be printed as the first 2 or 3 lines of the
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output file, assuming the <I>file</I> keyword was used. LAMMPS uses
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default values for each of these, so they do not need to be specified.
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</P>
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<P>By default, these header lines are as follows for <I>mode</I> = scalar:
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</P>
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<PRE># Time-averaged data for fix ID
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# TimeStep value1 value2 ...
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</PRE>
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<P>In the first line, ID is replaced with the fix-ID. In the second line
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the values are replaced with the appropriate fields from the fix
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ave/time command. There is no third line in the header of the file,
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so the <I>title3</I> setting is ignored when <I>mode</I> = scalar.
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</P>
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<P>By default, these header lines are as follows for <I>mode</I> = vector:
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</P>
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<PRE># Time-averaged data for fix ID
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# TimeStep Number-of-rows
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# Row value1 value2 ...
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</PRE>
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<P>In the first line, ID is replaced with the fix-ID. The second line
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describes the two values that are printed at the first of each section
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of output. In the third line the values are replaced with the
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appropriate fields from the fix ave/time command.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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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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are relevant to this fix.
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</P>
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<P>This fix produces a global scalar or vector or array which can be
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accessed by various <A HREF = "Section_howto.html#4_15">output commands</A>. The
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values can only be accessed on timesteps that are multiples of <I>Nfreq</I>
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since that is when averaging is performed.
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</P>
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<P>A scalar is produced if only a single input value is averaged and
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<I>mode</I> = scalar. A vector is produced if multiple input values are
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averaged for <I>mode</I> = scalar, or a single input value for <I>mode</I> =
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vector. In the first case, the length of the vector is the number of
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inputs. In the second case, the length of the vector is the same as
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the length of the input vector. An array is produced if multiple
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input values are averaged and <I>mode</I> = vector. The global array has #
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of rows = length of the input vectors and # of columns = number of
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inputs.
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</P>
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<P>If the fix prouduces a scalar or vector, then the scalar and each
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element of the vector may be either "intensive" or "extensive". If
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the fix produces an array, then all elements in the array will be
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either "intensive" or "extensive". If a compute or fix provides the
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value being time averaged, then the compute or fix determines whether
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the value is intensive or extensive; see the doc page for that compute
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or fix for further info. Values produced by a variable are whatever
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the variable calculates.
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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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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_atom.html">fix ave/atom</A>, <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A>, <A HREF = "fix_ave_histo.html">fix ave/histo</A>,
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<A HREF = "variable.html">variable</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<P>The option defaults are mode = scalar, ave = one, start = 0, no file
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output, title 1,2,3 = strings as described above, and no off settings
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for any input values.
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</P>
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</HTML>
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