lammps/doc/compute_reduce.txt

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:line

compute reduce command :h3

[Syntax:]

compute ID group-ID reduce mode input1 input2 ... :pre

ID, group-ID are documented in "compute"_compute.html command :ulb,l
reduce = style name of this compute command :l
mode = {sum} or {min} or {max} :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 = 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
:ule

[Examples:]

compute 1 all reduce 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 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 "thermo custom"_therml_style.html
command or "fix ave/time"_fix_ave_time.html command or by a
"variable"_variable.html command.  See "this
section"_Section_howto.html#4_15 of the documentation for an overview
of output options.

The reduction operation is specified by the {mode} setting.  The {sum}
option adds the per-atom quantities into a global total.  The {min} or
{max} options find the minimum or maximum value across all per-atom
quantities.

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.

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.  In the latter cases, the compute, fix, or
variable must produce per-atom quantities, not a global quantity.

"Computes"_compute.html that produce per-atom quantities are those
which have the word {atom} in their style name.  See the doc pages for
individual "fixes"_fix.html to determine which ones produce per-atom
quantities.  "Variables"_variable.html of style {atom} are the only
ones that can be used with this compute since all other variable
styles produce global quantities.

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:]

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} and {max} modes, the value(s) are
intensive.

[Restrictions:] none

[Related commands:]

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

[Default:] none