2007-02-21 08:18:41 +08:00
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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/spatial 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/spatial Nevery Nfreq dim origin delta file style args keyword value ...
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2007-02-21 08:18:41 +08:00
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</PRE>
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2007-02-21 08:49:36 +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/spatial = 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>dim = <I>x</I> or <I>y</I> or <I>z</I>
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<LI>origin = <I>lower</I> or <I>center</I> or <I>upper</I> or coordinate value (distance units)
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<LI>delta = thickness of spatial layers in dim (distance units)
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<LI>file = filename to write results to
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<LI>style = <I>density</I> or <I>atom</I> or <I>compute</I>
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<PRE> <I>density</I> arg = <I>mass</I> or <I>number</I>
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<I>mass</I> = compute mass density
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<I>number</I> = compute number density
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<I>atom</I> arg = <I>vx</I> or <I>vy</I> or <I>vz</I> or <I>fx</I> or <I>fy</I> or <I>fz</I>
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<I>compute</I> arg = compute-ID that calculates per-atom quantities
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</PRE>
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<LI>zero or more keyword/value pairs may be appended to the args
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<PRE>keyword = <I>norm</I> or <I>units</I>
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<I>norm</I> value = <I>all</I> or <I>sample</I>
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<I>units</I> value = <I>box</I> or <I>lattice</I>
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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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2007-02-28 06:00:03 +08:00
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<PRE>fix 1 all ave/spatial 10000 10000 z lower 2.0 centro.profile compute myCentro
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fix 1 flow ave/spatial 100 1000 y 0.0 1.0 vel.profile atom vx norm sample
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fix 1 flow ave/spatial 100 1000 y 0.0 1.0 dens.profile density mass
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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 per-atom quantities every few
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timesteps, average them by layer in a chosen dimension and over a
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longer timescale, and print the results to a file. This can be used
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to spatially average per-atom properties such as velocity or energy or
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a quantity calculated by an equation you define; see the <A HREF = "variable.html">variable
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atom</A> command.
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</P>
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<P>The <I>density</I> styles means to simply count the number of atoms in each
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layer, either by mass or number. The <I>atom</I> style allows an atom
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property such as x-velocity to be specified. The <I>compute</I> style
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allows specification of a <A HREF = "compute.html">compute</A> which will be invoked
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to calculate the desired property. The compute can be previously
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defined in the input script. Note that the "compute variable/atom"
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style allows you to calculate any quantity for an atom that can be
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specified by a <A HREF = "variable.html">variable atom</A> equation. Users can also
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write code for their own compute styles and <A HREF = "Section_modify.html">add them to
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LAMMPS</A>. Note that the <A HREF = "dump.html">dump custom</A>
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command can also be used to output per-atom quantities calculated by a
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compute.
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</P>
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<P>For the <I>compute</I> style, the fix ave/spatial style uses the per-atom
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scalar or vector calculated by the compute. See the <A HREF = "fix_ave_time.html">fix
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ave/time</A> command if you wish to time-average a
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global quantity, e.g. via a compute that temperature or pressure.
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</P>
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<P>In all cases, the calculated property is averaged over atoms in each
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layer, where the layers are in a particular <I>dim</I> and have a thickness
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given by <I>delta</I>. Every Nfreq steps, when a property is calculated
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for the first time (after a previous write), the number of layers and
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the layer boundaries are computed. Thus if the simlation box changes
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size during a simulation, the number of layers and their boundaries
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may also change. Layers are defined relative to a specified <I>origin</I>,
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which may be the lower/upper edge of the box (in <I>dim</I>) or its center
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point, or a specified coordinate value. Starting at the origin,
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sufficient layers are created in both directions to completely cover
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the box. On subsequent timesteps every atom is mapped to one of the
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layers. Atoms beyond the lowermost/uppermost layer are counted in the
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first/last layer.
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</P>
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2007-02-21 08:49:36 +08:00
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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for the layer thickness <I>delta</I> and <I>origin</I> if it is a coordinate
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value. A <I>box</I> value selects standard distance units as defined by
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the <A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or
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metal. A <I>lattice</I> value means the distance units are in lattice
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spacings. The <A HREF = "lattice.html">lattice</A> command must have been
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previously used to define the lattice spacing.
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</P>
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<P>The <I>Nevery</I> and <I>Nfreq</I> arguments specify how the property calculated
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for each layer is time-averaged. The property is calculated once each
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Nevery timesteps. It is averaged and output every Nfreq timesteps.
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Nfreq must be a multiple of Nevery. In the 2nd example above, the
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property is calculated every 100 steps. After 10 calculations, the
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average result is written to the file, once every 1000 steps.
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</P>
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<P>The <I>norm</I> keyword also affects how time-averaging is done. For an
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<I>all</I> setting, a layer quantity is summed over all atoms in all
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Nfreq/Nevery samples, as is the count of atoms in the layer. The
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printed value for the layer is Total-quantity / Total-count.
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In other words it is an average over the entire Nfreq timescale.
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</P>
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<P>For a <I>sample</I> setting, the quantity is summed over atoms for only a
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single sample, as is the count, and a "average sample value" is
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computed, i.e. Sample-quantity / Sample-count. The printed value for
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the layer is the average of the M "average sample values", where M =
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Nfreq/Nevery. In other words it is an average of an average.
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</P>
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<P>Each time info is written to the file, it is in the following format.
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A line with the timestep and number of layers is written. Then one
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line per layer is written, containing the layer ID (1-N), the
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coordinate of the center of the layer, the number of atoms in the
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layer, and one or more calculated values. The number of atoms and the
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value(s) are average quantities.
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</P>
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<P>If the <I>density</I> or <I>atom</I> keyword is used, or the <I>compute</I> keyword
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with a compute that calculates a single quantity per atom, then a
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single value will be printed for each layer. If the <I>compute</I> keyword
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is used with a compute that calculates N quantities per atom, then N
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values per line will be written, each of them averaged independently.
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</P>
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<P>For the <I>compute</I> keyword, the calculation performed by the compute in
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on the group defined by the compute. However, only atoms in the fix
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group are included in the layer averaging. LAMMPS prints a warning if
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the fix group and compute group do not match.
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</P>
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<P>Note that some computes perform costly calculations, involving use of
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or creation of neighbor lists. If the compute is invoked too often by
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fix ave/spatial, it can slow down a simulation.
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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_time.html">fix ave/time</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are norm = all and units = lattice.
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</P>
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</HTML>
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