forked from lijiext/lammps
86 lines
3.5 KiB
HTML
86 lines
3.5 KiB
HTML
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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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</CENTER>
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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 Nfreq compute-ID flag file
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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 property every this many timesteps
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<LI>Nfreq = write average property to file every this many steps
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<LI>compute-ID = ID of compute that performs the calculation
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<LI>flag = 0 for scalar quantity, 1 for vector quantity, 2 for both
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<LI>file = filename to write results to
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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 1000 myTemp 0 temp.stats
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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 quantities every few timesteps,
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average them over a longer timescale, and print the results to a file.
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This can be used to time-average any "compute" entity in LAMMPS such
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as a temperature or pressure.
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</P>
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<P>The <I>compute-ID</I> specifies a <A HREF = "compute.html">compute</A> which calculates
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the desired property. The compute can be previously defined in the
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input script. Or it can be a compute defined 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>. Users
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can also write code for their own compute 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>In all these cases, the fix ave/time style uses the global scalar or
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vector calculated by the compute. See the <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A> command if you wish to average
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spatially, e.g. via a compute that calculates per-atom quantities.
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</P>
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<P>The <I>Nevery</I> and <I>Nfreq</I> arguments specify how the property will be
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averaged. The property is calculated once each Nevery timesteps. It
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is averaged and output every Nfreq timesteps. Nfreq must be a
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multiple of Nevery. In the example above, the property is calculated
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every 100 steps. After 10 calculations, the average result is written
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to the file, once every 1000 steps.
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</P>
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<P>The <I>flag</I> argument chooses whether the scalar and/or vector
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calculation of the compute is invoked. The former computes a single
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global value. The latter computes N global values, where N is defined
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by the compute, e.g. 6 pressure tensor components. In the vector
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case, each of the N values is averaged independently and N values are
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written to the file at each output.
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</P>
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<P>Since the calculation is performed by the compute which stores its own
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"group" definition, the group specified for the fix is ignored.
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LAMMPS prints a warning if the fix group and compute group do not
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match.
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</P>
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<P>If the compute calculates pressure, it will cause the force
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computations performed by LAMMPS (pair, bond, angle, etc) to calculate
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virial terms each Nevery timesteps. If this is more frequent than
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thermodynamic output, this adds extra cost to a simulation. However,
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if a constant pressure simulation is being run (<A HREF = "fix_npt.html">fix npt</A>
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or <A HREF = "fix_nph.html">fix nph</A>), LAMMPS is already calculating virial terms
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for the pressure every timestep.
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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>
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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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