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222 lines
10 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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<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 = calculate properties 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 averages are computed
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<LI>one or more 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 value calculated by a compute with ID
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c_ID[N] = Nth component of global vector calculated by a compute with ID
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f_ID = global scalar value calculated by a fix with ID
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f_ID[N] = Nth component of global vector 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>file</I> or <I>ave</I>
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<PRE> <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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</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 myTemp thermo_temp file temp.profile
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fix 1 all ave/time 100 5 1000 thermo_press<B>2</B> ave window 20
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fix 1 all ave/time 1 100 1000 f_indent f_indent<B>1</B> file temp.indent
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Calculate one or more instantaneous global quantities every few
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timesteps, and average them over longer timescales. The resulting
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averages can be used by other <A HREF = "Section_howto.html#4_15">output
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commands</A> such as <A HREF = "thermo_style.html">thermo_style
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custom</A>, and can also be written to a file. If no
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averaging is done, this command is a convenient way to simply write
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one or more desired quantities to a separate file.
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</P>
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<P>Each listed value is averaged independently. If written to a file,
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then over time, one column of numbers is produced for each value. The
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group specified with the command is ignored, since calculations are
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performed by computes and fixes which store their own "group"
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definition,
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</P>
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<P>Each listed 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 quantity. If you wish
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to spatial- or time-average per-atom quantities from a compute, fix,
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or variable, then see the <A HREF = "fix_ave_spatial.html">fix ave/spatial</A> or
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<A HREF = "fix_ave_atom.html">fix ave/atom</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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<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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preceding portion of the simulation every <I>Nevery</I> timesteps.
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<I>Nfreq</I> must be a multiple of <I>Nevery</I> and <I>Nevery</I> must be non-zero
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even if <I>Nrepeat</I> is 1.
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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 no bracketed term is
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appended, the global scalar calculated by the compute is used. If a
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bracketed term is appended, the Nth vector value calculated by the
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compute is used. Note that there is a <A HREF = "compute_reduce.html">compute
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reduce</A> command which can sum per-atom quantities
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into a global scalar or vector which can thus be accessed by fix
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ave/time. Or it can be a compute defined not in your input script,
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but by <A HREF = "thermo_style.html">thermodynamic output</A> or other fixes such as
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<A HREF = "fix_nvt.html">fix nvt</A> or <A HREF = "fix_temp_rescale.html">fix temp/rescale</A>.
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See the doc pages for these commands which give the IDs of these
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computes. Users can also write code for their own compute styles and
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<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 global scalar calculated by the fix is used. If a
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bracketed term is appended, the Nth vector value calculated by the fix
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is used. Note that some fixes only produce their values on certain
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timesteps, which must be compatible with <I>Nevery</I>, else an error will
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result. Users can also write code for their own fix styles and <A HREF = "Section_modify.html">add
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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. Only equal-style
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variables can be referenced. See the <A HREF = "variable.html">variable</A> command
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for details. 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 generating 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>The <I>file</I> keyword allows a filename to be specified. Each timestamp,
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one quantity is written to the file for each value specified in the
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fix ave/time command. The file is in a self-explanatory text format.
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</P>
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<P>The <I>ave</I> keyword determines how the scalar and/or vector values
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produced every <I>Nfreq</I> steps are averaged with values produced on
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previous steps that were multiples of <I>Nfreq</I>, before they are
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accessed by another output 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 this value is 0.0, so setting <I>start</I> to
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a larger value can avoid including a 0.0 in a running or windowed
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average.
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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 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 global values can only be accessed on timesteps that are multiples
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of <I>Nfreq</I> since that is when averaging is performed. Each value
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(scalar or vector component) calculated by this fix may be either
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"intensive" or "extensive". Intensive means the value is independent
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of the number of atoms in the simulation. Extensive means the value
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scales with the number of atoms in the simulation. If a compute or
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fix provides the value being time averaged, then the compute or fix
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determines whether the value is intensive or extensive; see the doc
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page for that compute or fix for further info. Values produced by a
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variable are whatever 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>,
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<A HREF = "variable.html">variable</A>, <A HREF = "fix_ave_spatial.html">fix ave/spatial</A>, <A HREF = "fix_ave_atom.html">fix
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ave/atom</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 no file output, ave = one, and start = 0.
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</P>
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</HTML>
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