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<HTML>
<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>
</CENTER>
<HR>
<H3>compute reduce command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID reduce mode input1 input2 ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>reduce = style name of this compute command
<LI>mode = <I>sum</I> or <I>min</I> or <I>max</I>
<LI>one or more inputs can be listed
<LI>input = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[N], f_ID, f_ID[N], v_name
<PRE> x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
c_ID = per-atom scalar value calculated by a compute with ID
c_ID[N] = Nth component of per-atom vector calculated by a compute with ID
f_ID = per-atom scalar value calculated by a fix with ID
f_ID[N] = Nth component of per-atom vector calculated by a fix with ID
v_name = per-atom value calculated by an atom-style variable with name
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 all reduce sum c_force
compute 2 all reduce min c_press<B>2</B> f_ave v_myKE
</PRE>
<P><B>Description:</B>
</P>
<P>Define a calculation that "reduces" one or more per-atom inputs across
all atoms in the group to yield a single global scalar for each listed
input. The resulting value(s) can be accessed by any command that
uses global computes, e.g. the <A HREF = "therml_style.html">thermo custom</A>
command or <A HREF = "fix_ave_time.html">fix ave/time</A> command or by a
<A HREF = "variable.html">variable</A> command. See <A HREF = "Section_howto.html#4_15">this
section</A> of the documentation for an overview
of output options.
</P>
<P>The reduction operation is specified by the <I>mode</I> setting. The <I>sum</I>
option adds the per-atom quantities into a global total. The <I>min</I> or
<I>max</I> options find the minimum or maximum value across all per-atom
quantities.
</P>
<P>Each listed input is operated on independently. The group specified
with the command means only atoms within the group contribute to the
result. Note that the input that produces the per-atom quantities may
define its own group which affects the quantities it returns. For
example, if a per-atom compute is used as an input, it will generate
values of 0.0 for atoms that are not in the group specified for that
compute.
</P>
<P>Each listed input can be an atom attribute (position, velocity, force
component) or can be the result of a <A HREF = "compute.html">compute</A> or
<A HREF = "fix.html">fix</A> or the evaluation of an atom-style
<A HREF = "variable.html">variable</A>. In the latter cases, the compute, fix, or
variable must produce per-atom quantities, not a global quantity.
</P>
<P><A HREF = "compute.html">Computes</A> that produce per-atom quantities are those
which have the word <I>atom</I> in their style name. See the doc pages for
individual <A HREF = "fix.html">fixes</A> to determine which ones produce per-atom
quantities. <A HREF = "variable.html">Variables</A> of style <I>atom</I> are the only
ones that can be used with this compute since all other variable
styles produce global quantities.
</P>
<P>If a single input is specified this compute produces a global scalar
value. If multiple inputs are specified, this compute produces a
vector of global values, the length of which is equal to the number of
inputs specified.
</P>
<P>The value(s) produced by this compute are all "extensive", meaning
their value scales linearly with the number of atoms involved. If
normalized values are desired, this compute can be accessed by the
<A HREF = "thermo_style.html">thermo_style custom</A> command with <A HREF = "thermo_modify.html">thermo_modify
norm yes</A> set as an option. Or it can be accessed
by a <A HREF = "variable.html">variable</A> that divides by the appropriate atom
count.
</P>
<P><B>Output info:</B>
</P>
<P>The scalar and vector values calculated by this compute are
"extensive", meaning they scale with the number of atoms in the
simulation.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute.html">compute</A>, <A HREF = "fix.html">fix</A>, <A HREF = "variable.html">variable</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>