2008-02-20 23:00:04 +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>compute reduce command
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</H3>
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2009-04-28 22:56:27 +08:00
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<H3>compute reduce/region command
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</H3>
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2008-02-20 23:00:04 +08:00
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<P><B>Syntax:</B>
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</P>
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2010-01-01 05:08:23 +08:00
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<PRE>compute ID group-ID style arg mode input1 input2 ... keyword args ...
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2008-02-20 23:00:04 +08:00
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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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2009-04-28 22:56:27 +08:00
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<LI>style = <I>reduce</I> or <I>reduce/region</I>
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2008-02-20 23:00:04 +08:00
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2009-04-28 22:56:27 +08:00
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<PRE> <I>reduce</I> arg = none
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<I>reduce/region</I> arg = region-ID
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region-ID = ID of region to use for choosing atoms
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</PRE>
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2009-12-10 05:20:59 +08:00
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<LI>mode = <I>sum</I> or <I>min</I> or <I>max</I> or <I>ave</I>
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2008-02-20 23:00:04 +08:00
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<LI>one or more inputs can be listed
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<LI>input = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[N], f_ID, f_ID[N], v_name
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<PRE> x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
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2010-08-18 05:58:29 +08:00
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c_ID = per-atom or local vector calculated by a compute with ID
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c_ID[I] = Ith column of per-atom or local array calculated by a compute with ID
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f_ID = per-atom or local vector calculated by a fix with ID
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f_ID[I] = Ith column of per-atom or local array calculated by a fix with ID
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2009-12-05 05:04:00 +08:00
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v_name = per-atom vector calculated by an atom-style variable with name
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2008-02-20 23:00:04 +08:00
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</PRE>
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<LI>zero or more keyword/args pairs may be appended
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<LI>keyword = <I>replace</I>
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<PRE> <I>replace</I> args = vec1 vec2
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vec1 = reduced value from this input vector will be replaced
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vec2 = replace it with vec1[N] where N is index of max/min value from vec2
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</PRE>
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2008-02-20 23:00:04 +08:00
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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 reduce sum c_force
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2009-04-28 22:56:27 +08:00
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compute 1 all reduce/region subbox sum c_force
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2010-01-01 05:08:23 +08:00
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compute 2 all reduce min c_press<B>2</B> f_ave v_myKE
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compute 3 fluid reduce max c_index<B>1</B> c_index<B>2</B> c_dist replace 1 3 replace 2 3
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2008-02-20 23:00:04 +08:00
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</PRE>
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<P><B>Description:</B>
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</P>
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2009-12-10 05:20:59 +08:00
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<P>Define a calculation that "reduces" one or more vector inputs into
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2010-08-18 05:58:29 +08:00
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scalar values, one per listed input. The inputs can be per-atom or
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local quantities; they cannot be global quantities. Atom attributes
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are per-atom quantities, <A HREF = "compute.html">computes</A> and <A HREF = "fix.html">fixes</A>
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may generate any of the three kinds of quantities, and <A HREF = "variable.html">atom-style
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variables</A> generate per-atom quantities. See the
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<A HREF = "variable">variable</A> command and its special functions which can
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perform the same operations as the compute reduce command on global
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vectors.
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2008-02-20 23:00:04 +08:00
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</P>
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<P>The reduction operation is specified by the <I>mode</I> setting. The <I>sum</I>
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2009-12-10 05:20:59 +08:00
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option adds the values in the vector into a global total. The <I>min</I>
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or <I>max</I> options find the minimum or maximum value across all vector
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values. The <I>ave</I> setting adds the vector values into a global total,
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then divides by the number of values in the vector.
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</P>
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<P>Each listed input is operated on independently. For per-atom inputs,
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the group specified with this command means only atoms within the
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group contribute to the result. For per-atom inputs, if the compute
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reduce/region command is used, the atoms must also currently be within
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the region. Note that an input that produces per-atom quantities may
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define its own group which affects the quantities it returns. For
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example, if a compute is used as an input which generates a per-atom
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vector, it will generate values of 0.0 for atoms that are not in the
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2009-04-28 22:56:27 +08:00
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group specified for that compute.
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2008-02-20 23:00:04 +08:00
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</P>
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<P>Each listed input can be an atom attribute (position, velocity, force
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component) or 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 atom-style
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2009-12-10 05:20:59 +08:00
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<A HREF = "variable.html">variable</A>.
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</P>
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2009-12-17 01:12:09 +08:00
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<P>The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
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self-explanatory. Note that other atom attributes can be used as
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inputs to this fix by using the <A HREF = "compute_property_atom.html">compute
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property/atom</A> command and then specifying
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an input value from that compute.
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2009-12-10 05:20:59 +08:00
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</P>
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<P>If a value begins with "c_", a compute ID must follow which has been
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2010-08-18 05:58:29 +08:00
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previously defined in the input script. Computes can generate
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per-atom or local quantities. See the individual
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2009-12-10 05:20:59 +08:00
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<A HREF = "compute.html">compute</A> doc page for details. If no bracketed integer
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is appended, the vector calculated by the compute is used. If a
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bracketed interger is appended, the Ith column of the array calculated
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by the compute is used. Users can also write code for their own
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compute styles and <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. Fixes can generate per-atom
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or local quantities. 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 reduce
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2009-12-10 05:20:59 +08:00
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references the values, else an error results. If no bracketed integer
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is 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 value begins with "v_", a variable name must follow which has
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been previously defined in the input script. It must be an
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<A HREF = "variable.html">atom-style variable</A>. Atom-style variables can
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reference thermodynamic keywords and various per-atom attributes, or
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invoke other computes, fixes, or variables when they are evaluated, so
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this is a very general means of generating per-atom quantities to
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reduce.
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2008-02-20 23:00:04 +08:00
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</P>
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2010-01-01 05:08:23 +08:00
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<HR>
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<P>If the <I>replace</I> keyword is used, two indices <I>vec1</I> and <I>vec2</I> are
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specified, where each index ranges from 1 to the # of input values.
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The replace keyword can only be used if the <I>mode</I> is <I>min</I> or <I>max</I>.
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It works as follows. A min/max is computed as usual on the <I>vec2</I>
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input vector. The index N of that value within <I>vec2</I> is also stored.
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Then, instead of performing a min/max on the <I>vec1</I> input vector, the
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stored index is used to select the Nth element of the <I>vec1</I> vector.
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</P>
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<P>Thus, for example, if you wish to use this compute to find the bond
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with maximum stretch, you can do it as follows:
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</P>
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<PRE>compute 1 all property/local batom1 batom2
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compute 2 all bond/local dist
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compute 3 all reduce max c_1[1] c_1[2] c_2 replace 1 3 replace 2 3
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thermo_style custom step temp c_3[1] c_3[2] c_3[3]
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</PRE>
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<P>The first two input values in the compute reduce command are vectors
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with the IDs of the 2 atoms in each bond, using the <A HREF = "compute_property_local.html">compute
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property/local</A> command. The last input
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value is bond distance, using the <A HREF = "compute_bond_local.html">compute
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bond/local</A> command. Instead of taking the
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max of the two atom ID vectors, which does not yield useful
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information in this context, the <I>replace</I> keywords will extract the
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atom IDs for the two atoms in the bond of maximum stretch. These atom
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IDs and the bond stretch will be printed with thermodynamic output.
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</P>
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<HR>
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2008-02-20 23:00:04 +08:00
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<P>If a single input is specified this compute produces a global scalar
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value. If multiple inputs are specified, this compute produces a
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global vector of values, the length of which is equal to the number of
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2008-02-20 23:00:04 +08:00
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inputs specified.
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</P>
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2008-02-20 23:42:58 +08:00
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<P>As discussed below, for <I>sum</I> mode, the value(s) produced by this
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compute are all "extensive", meaning their value scales linearly with
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the number of atoms involved. If normalized values are desired, this
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compute can be accessed by the <A HREF = "thermo_style.html">thermo_style custom</A>
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command with <A HREF = "thermo_modify.html">thermo_modify norm yes</A> set as an
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option. Or it can be accessed by a <A HREF = "variable.html">variable</A> that
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divides by the appropriate atom count.
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2008-02-20 23:00:04 +08:00
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</P>
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2010-08-18 05:58:29 +08:00
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<HR>
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2008-02-20 23:00:04 +08:00
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<P><B>Output info:</B>
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</P>
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2010-08-18 05:58:29 +08:00
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<P>This compute calculates a global scalar if a single input value is
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specified or a global vector of length N where N is the number of
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inputs, and which can be accessed by indices 1 to N. These values can
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be used by any command that uses global scalar or vector values from a
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compute as input. See <A HREF = "Section_howto.html#4_15">this section</A> for an
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overview of LAMMPS output options.
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2009-12-04 07:58:11 +08:00
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</P>
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2010-08-18 05:58:29 +08:00
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<P>All the scalar or vector values calculated by this compute are
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"intensive", except when the <I>sum</I> mode is used on per-atom or local
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vectors, in which case the calculated values are "extensive".
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2008-02-20 23:00:04 +08:00
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</P>
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2010-01-24 07:20:05 +08:00
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<P>The scalar or vector values will be in whatever <A HREF = "units.html">units</A> the
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quantities being reduced are in.
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</P>
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2008-02-20 23:00:04 +08:00
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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 = "variable.html">variable</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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