forked from lijiext/lammps
207 lines
8.5 KiB
Plaintext
207 lines
8.5 KiB
Plaintext
.. index:: compute reduce
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compute reduce command
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======================
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compute reduce/region command
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=============================
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Syntax
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""""""
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.. parsed-literal::
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compute ID group-ID style arg mode input1 input2 ... keyword args ...
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* style = *reduce* or *reduce/region*
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.. parsed-literal::
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*reduce* arg = none
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*reduce/region* arg = region-ID
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region-ID = ID of region to use for choosing atoms
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* mode = *sum* or *min* or *max* or *ave* or *sumsq* or *avesq*
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* one or more inputs can be listed
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* 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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.. parsed-literal::
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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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* zero or more keyword/args pairs may be appended
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* keyword = *replace*
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.. parsed-literal::
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*replace* 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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Examples
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""""""""
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.. parsed-literal::
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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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Description
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"""""""""""
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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, :doc:`computes <compute>` and :doc:`fixes <fix>`
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may generate any of the three kinds of quantities, and :doc:`atom-style variables <variable>` generate per-atom quantities. See the
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:doc:`variable <variable>` 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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The reduction operation is specified by the *mode* setting. The *sum*
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option adds the values in the vector into a global total. The *min*
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or *max* options find the minimum or maximum value across all vector
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values. The *ave* setting adds the vector values into a global total,
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then divides by the number of values in the vector. The *sumsq*
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option sums the square of the values in the vector into a global
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total. The *avesq* setting does the same as *sumsq*\ , 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.
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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.
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Each listed input can be an atom attribute (position, velocity, force
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component) or can be the result of a :doc:`compute <compute>` or
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:doc:`fix <fix>` or the evaluation of an atom-style
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:doc:`variable <variable>`.
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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 :doc:`compute property/atom <compute_property_atom>` command and then specifying
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an input value from that compute.
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If a value begins with "c_", 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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:doc:`compute <compute>` 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 :doc:`add them to LAMMPS <Section_modify>`.
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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 :doc:`fix <fix>` 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 :doc:`add them to LAMMPS <Section_modify>`.
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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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:doc:`atom-style variable <variable>`. 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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----------
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If the *replace* keyword is used, two indices *vec1* and *vec2* 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 *mode* is *min* or *max*\ .
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It works as follows. A min/max is computed as usual on the *vec2*
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input vector. The index N of that value within *vec2* is also stored.
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Then, instead of performing a min/max on the *vec1* input vector, the
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stored index is used to select the Nth element of the *vec1* vector.
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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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.. parsed-literal::
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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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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 :doc:`compute property/local <compute_property_local>` command. The last input
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value is bond distance, using the :doc:`compute bond/local <compute_bond_local>` 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 *replace* 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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----------
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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.
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As discussed below, for the *sum* and *sumsq* 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 :doc:`thermo_style custom <thermo_style>` command with :doc:`thermo_modify norm yes <thermo_modify>` set as an option. Or it can be accessed by a
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:doc:`variable <variable>` that divides by the appropriate atom count.
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----------
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**Output info:**
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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 :ref:`Section_howto 15 <howto_15>`
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for an overview of LAMMPS output options.
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All the scalar or vector values calculated by this compute are
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"intensive", except when the *sum* or *sumsq* 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".
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The scalar or vector values will be in whatever :doc:`units <units>` the
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quantities being reduced are in.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`compute <compute>`, :doc:`fix <fix>`, :doc:`variable <variable>`
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**Default:** none
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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