lammps/doc/compute_reduce.txt

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compute reduce command :h3
compute reduce/region command :h3

[Syntax:]

compute ID group-ID style arg mode input1 input2 ... :pre

ID, group-ID are documented in "compute"_compute.html command :ulb,l
style = {reduce} or {reduce/region} :l
  {reduce} arg = none
  {reduce/region} arg = region-ID
    region-ID = ID of region to use for choosing atoms :pre
mode = {sum} or {min} or {max} or {ave} :l
one or more inputs can be listed :l
input = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID\[N\], f_ID, f_ID\[N\], v_name :l
  x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
  c_ID = vector calculated by a compute with ID
  c_ID\[I\] = Ith column of array calculated by a compute with ID
  f_ID = vector calculated by a fix with ID
  f_ID\[I\] = Ith column of array calculated by a fix with ID
  v_name = per-atom vector calculated by an atom-style variable with name :pre
:ule

[Examples:]

compute 1 all reduce sum c_force
compute 1 all reduce/region subbox sum c_force
compute 2 all reduce min c_press[2] f_ave v_myKE :pre

[Description:]

Define a calculation that "reduces" one or more vector inputs into
scalar values, one per listed input.  The inputs can be global,
per-atom, or local quantities.  Atom attributes are per-atom
quantities, "computes"_compute.html and "fixes"_fix.html may generate
any of the three kinds of quantities, and "atom-style
variables"_variable.html generate per-atom quantities.

The reduction operation is specified by the {mode} setting.  The {sum}
option adds the values in the vector into a global total.  The {min}
or {max} options find the minimum or maximum value across all vector
values.  The {ave} setting adds the vector values into a global total,
then divides by the number of values in the vector.

Each listed input is operated on independently.  For per-atom inputs,
the group specified with this command means only atoms within the
group contribute to the result.  For per-atom inputs, if the compute
reduce/region command is used, the atoms must also currently be within
the region.  Note that an input that produces per-atom quantities may
define its own group which affects the quantities it returns.  For
example, if a compute is used as an input which generates a per-atom
vector, it will generate values of 0.0 for atoms that are not in the
group specified for that compute.

Each listed input can be an atom attribute (position, velocity, force
component) or can be the result of a "compute"_compute.html or
"fix"_fix.html or the evaluation of an atom-style
"variable"_variable.html.

Atom attributes are per-atom inputs.

If a value begins with "c_", a compute ID must follow which has been
previously defined in the input script.  Computes can generate global,
per-atom, or local quantities.  See the individual
"compute"_compute.html doc page for details.  If no bracketed integer
is appended, the vector calculated by the compute is used.  If a
bracketed interger is appended, the Ith column of the array calculated
by the compute is used.  Users can also write code for their own
compute styles and "add them to LAMMPS"_Section_modify.html.

If a value begins with "f_", a fix ID must follow which has been
previously defined in the input script.  Fixes can generate global,
per-atom, or local quantities.  See the individual "fix"_fix.html doc
page for details.  Note that some fixes only produce their values on
certain timesteps, which must be compatible with when compute reduce
references the values, else an error results.  If no bracketed integer
is appended, the vector calculated by the fix is used.  If a bracketed
integer is appended, the Ith column of the array calculated by the fix
is used.  Users can also write code for their own fix style and "add
them to LAMMPS"_Section_modify.html.

If a value begins with "v_", a variable name must follow which has
been previously defined in the input script.  It must be an
"atom-style variable"_variable.html.  Atom-style variables can
reference thermodynamic keywords and various per-atom attributes, or
invoke other computes, fixes, or variables when they are evaluated, so
this is a very general means of generating per-atom quantities to
reduce.

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.

As discussed below, for {sum} mode, 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 "thermo_style custom"_thermo_style.html
command with "thermo_modify norm yes"_thermo_modify.html set as an
option.  Or it can be accessed by a "variable"_variable.html that
divides by the appropriate atom count.

[Output info:]

This compute calculates a global scalar or global vector of length N
where N is the number of inputs, and which can be accessed by indices
1-N.  These values can be used by any command that uses global scalar
or vector values from a compute as input.  See "this
section"_Section_howto.html#4_15 for an overview of LAMMPS output
options.

For {sum} mode, the scalar and vector values calculated by this
compute are "extensive", meaning they scale with the number of atoms
in the simulation.  For {min} or {max} or {ave} modes, the value(s)
are intensive.

[Restrictions:] none

[Related commands:]

"compute"_compute.html, "fix"_fix.html, "variable"_variable.html

[Default:] none