forked from lijiext/lammps
125 lines
5.1 KiB
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125 lines
5.1 KiB
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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>compute slice command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>compute ID group-ID slice Nstart Nstop Nskip input1 input2 ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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<LI>slice = style name of this compute command
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<LI>Nstart = starting index within input vector(s)
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<LI>Nstop = stopping index within input vector(s)
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<LI>Nskip = extract every Nskip elements from input vector(s)
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<LI>input = c_ID, c_ID[N], f_ID, f_ID[N]
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<PRE> c_ID = global vector calculated by a compute with ID
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c_ID[I] = Ith column of global array calculated by a compute with ID
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f_ID = global vector calculated by a fix with ID
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f_ID[I] = Ith column of global array calculated by a fix with ID
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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>compute 1 all slice 1 100 10 c_msdmol[4]
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compute 1 all slice 301 400 1 c_msdmol[4]
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Define a calculation that "slices" one or more vector inputs into
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smaller vectors, one per listed input. The inputs can be global
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quantities; they cannot be per-atom or local quantities.
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<A HREF = "compute.html">Computes</A> and <A HREF = "fix.html">fixes</A> may generate any of the
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three kinds of quantities. <A HREF = "variable.html">Variables</A> do not generate
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global vectors. The group specified with this command is ignored.
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</P>
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<P>The values extracted from the input vector(s) are determined by the
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<I>Nstart</I>, <I>Nstop</I>, and <I>Nskip</I> parameters. The elements of an input
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vector of length N are indexed from 1 to N. Starting at element
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<I>Nstart</I>, every Mth element is extracted, where M = <I>Nskip</I>, until
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element <I>Nstop</I> is reached. The extracted quantities are stored as a
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vector, which is typically shorter than the input vector.
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</P>
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<P>Each listed input is operated on independently to produce one output
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vector. Each listed input must be a global vector or column of a
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global array calculated by another <A HREF = "compute.html">compute</A> or
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<A HREF = "fix.html">fix</A>.
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</P>
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<P>If an input value begins with "c_", a compute ID must follow which has
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been previously defined in the input script and which generates a
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global vector or array. See the individual <A HREF = "compute.html">compute</A> doc
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page for details. If no bracketed integer is appended, the vector
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calculated by the compute is used. If a bracketed integer is
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appended, the Ith column of the array calculated by the compute is
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used. Users can also write code for their own compute 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 "f_", a fix ID must follow which has been
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previously defined in the input script and which generates a global
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vector or array. See the individual <A HREF = "fix.html">fix</A> doc page for
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details. Note that some fixes only produce their values on certain
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timesteps, which must be compatible with when compute slice references
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the values, else an error results. If no bracketed integer is
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appended, the vector calculated by the fix is used. If a bracketed
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integer is appended, the Ith column of the array calculated by the fix
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is used. Users can also write code for their own fix style 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 single input is specified this compute produces a global vector,
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even if the length of the vector is 1. If multiple inputs are
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specified, then a global array of values is produced, with the number
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of columns equal to the number of inputs specified.
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</P>
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<HR>
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<P><B>Output info:</B>
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</P>
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<P>This compute calculates a global vector if a single input value is
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specified or a global array with N columns where N is the number of
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inputs. The length of the vector or the number of rows in the array
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is equal to the number of values extracted from each input vector.
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These values can be used by any command that uses global vector or
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array values from a compute as input. See <A HREF = "Section_howto.html#howto_15">this
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section</A> for an overview of LAMMPS output
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options.
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</P>
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<P>The vector or array values calculated by this compute are simply
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copies of values generated by computes or fixes that are input vectors
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to this compute. If there is a single input vector of intensive
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and/or extensive values, then each value in the vector of values
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calculated by this compute will be "intensive" or "extensive",
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depending on the corresponding input value. If there are multiple
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input vectors, and all the values in them are intensive, then the
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array values calculated by this compute are "intensive". If there are
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multiple input vectors, and any value in them is extensive, then the
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array values calculated by this compute are "extensive".
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</P>
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<P>The vector or array values will be in whatever <A HREF = "units.html">units</A> the
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input quantities are in.
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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.html">fix</A>, <A HREF = "compute_reduce.html">compute
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reduce</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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