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<div class="section" id="compute-reduce-command">
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<span id="index-0"></span><h1>compute reduce command<a class="headerlink" href="#compute-reduce-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="compute-reduce-region-command">
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<h1>compute reduce/region command<a class="headerlink" href="#compute-reduce-region-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>compute ID group-ID style arg mode input1 input2 ... keyword args ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><em>compute</em></a> command</li>
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<li>style = <em>reduce</em> or <em>reduce/region</em></li>
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</ul>
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<pre class="literal-block">
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<em>reduce</em> arg = none
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<em>reduce/region</em> 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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<ul class="simple">
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<li>mode = <em>sum</em> or <em>min</em> or <em>max</em> or <em>ave</em> or <em>sumsq</em> or <em>avesq</em></li>
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<li>one or more inputs can be listed</li>
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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</li>
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</ul>
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<div class="highlight-python"><div class="highlight"><pre>x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
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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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v_name = per-atom vector calculated by an atom-style variable with name
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</pre></div>
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</div>
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<ul class="simple">
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<li>zero or more keyword/args pairs may be appended</li>
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<li>keyword = <em>replace</em></li>
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</ul>
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<pre class="literal-block">
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<em>replace</em> 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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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>compute 1 all reduce sum c_force
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compute 1 all reduce/region subbox sum c_force
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compute 2 all reduce min c_press[2] f_ave v_myKE
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compute 3 fluid reduce max c_index[1] c_index[2] c_dist replace 1 3 replace 2 3
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Define a calculation that “reduces” one or more vector inputs into
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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 class="reference internal" href="compute.html"><em>computes</em></a> and <a class="reference internal" href="fix.html"><em>fixes</em></a>
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may generate any of the three kinds of quantities, and <a class="reference internal" href="variable.html"><em>atom-style variables</em></a> generate per-atom quantities. See the
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<a class="reference external" 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.</p>
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<p>The reduction operation is specified by the <em>mode</em> setting. The <em>sum</em>
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option adds the values in the vector into a global total. The <em>min</em>
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or <em>max</em> options find the minimum or maximum value across all vector
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values. The <em>ave</em> setting adds the vector values into a global total,
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then divides by the number of values in the vector. The <em>sumsq</em>
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option sums the square of the values in the vector into a global
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total. The <em>avesq</em> setting does the same as <em>sumsq</em>, then divdes the
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sum of squares by the number of values. The last two options can be
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useful for calculating the variance of some quantity, e.g. variance =
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sumsq - ave^2.</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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group specified for that compute.</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 class="reference internal" href="compute.html"><em>compute</em></a> or
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<a class="reference internal" href="fix.html"><em>fix</em></a> or the evaluation of an atom-style
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<a class="reference internal" href="variable.html"><em>variable</em></a>.</p>
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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 class="reference internal" href="compute_property_atom.html"><em>compute property/atom</em></a> command and then specifying
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an input value from that compute.</p>
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<p>If a value begins with “<a href="#id1"><span class="problematic" id="id2">c_</span></a>”, a compute ID must follow which has been
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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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<a class="reference internal" href="compute.html"><em>compute</em></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 integer 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 class="reference internal" href="Section_modify.html"><em>add them to LAMMPS</em></a>.</p>
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<p>If a value begins with “<a href="#id3"><span class="problematic" id="id4">f_</span></a>”, 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 class="reference internal" href="fix.html"><em>fix</em></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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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 class="reference internal" href="Section_modify.html"><em>add them to LAMMPS</em></a>.</p>
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<p>If a value begins with “<a href="#id5"><span class="problematic" id="id6">v_</span></a>”, 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 class="reference internal" href="variable.html"><em>atom-style variable</em></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.</p>
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<hr class="docutils" />
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<p>If the <em>replace</em> keyword is used, two indices <em>vec1</em> and <em>vec2</em> 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 <em>mode</em> is <em>min</em> or <em>max</em>.
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It works as follows. A min/max is computed as usual on the <em>vec2</em>
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input vector. The index N of that value within <em>vec2</em> is also stored.
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Then, instead of performing a min/max on the <em>vec1</em> input vector, the
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stored index is used to select the Nth element of the <em>vec1</em> vector.</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:</p>
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<div class="highlight-python"><div class="highlight"><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></div>
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</div>
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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 class="reference internal" href="compute_property_local.html"><em>compute property/local</em></a> command. The last input
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value is bond distance, using the <a class="reference internal" href="compute_bond_local.html"><em>compute bond/local</em></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 <em>replace</em> 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.</p>
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<hr class="docutils" />
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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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inputs specified.</p>
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<p>As discussed below, for the <em>sum</em> and <em>sumsq</em> modes, the value(s)
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produced by this compute are all “extensive”, meaning their value
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scales linearly with the number of atoms involved. If normalized
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values are desired, this compute can be accessed by the <a class="reference internal" href="thermo_style.html"><em>thermo_style custom</em></a> command with <a class="reference internal" href="thermo_modify.html"><em>thermo_modify norm yes</em></a> set as an option. Or it can be accessed by a
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<a class="reference internal" href="variable.html"><em>variable</em></a> that divides by the appropriate atom count.</p>
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<hr class="docutils" />
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<p><strong>Output info:</strong></p>
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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 class="reference internal" href="Section_howto.html#howto-15"><span>Section_howto 15</span></a>
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for an overview of LAMMPS output options.</p>
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<p>All the scalar or vector values calculated by this compute are
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“intensive”, except when the <em>sum</em> or <em>sumsq</em> modes are used on
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per-atom or local vectors, in which case the calculated values are
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“extensive”.</p>
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<p>The scalar or vector values will be in whatever <a class="reference internal" href="units.html"><em>units</em></a> the
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quantities being reduced are in.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="compute.html"><em>compute</em></a>, <a class="reference internal" href="fix.html"><em>fix</em></a>, <a class="reference internal" href="variable.html"><em>variable</em></a></p>
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<p><strong>Default:</strong> none</p>
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