# compute reduce command

# compute reduce/region command

## Syntax

``` LAMMPS
compute ID group-ID style arg mode input1 input2 ... keyword args ...
```

-   ID, group-ID are documented in [compute](compute) command

-   style = *reduce* or *reduce/region*

        *reduce* arg = none
        *reduce/region* arg = region-ID
          region-ID = ID of region to use for choosing atoms

-   mode = *sum* or *min* or *minabs* or *max* or *maxabs* or *ave* or
    *sumsq* or *avesq* or *sumabs* or *aveabs*

-   one or more inputs can be listed

-   input = *x* or *y* or *z* or *vx* or *vy* or *vz* or *fx* or *fy* or
    *fz* or c_ID or c_ID\[N\] or f_ID or f_ID\[N\] or v_name

        *x*,\ *y*,\ *z*,\ *vx*,\ *vy*,\ *vz*,\ *fx*,\ *fy*,\ *fz* = atom attribute (position, velocity, force component)
        c_ID = per-atom or local vector calculated by a compute with ID
        c_ID[I] = Ith column of per-atom or local array calculated by a compute with ID, I can include wildcard (see below)
        f_ID = per-atom or local vector calculated by a fix with ID
        f_ID[I] = Ith column of per-atom or local array calculated by a fix with ID, I can include wildcard (see below)
        v_name = per-atom vector calculated by an atom-style variable with name

-   zero or more keyword/args pairs may be appended

-   keyword = *replace*

        *replace* args = vec1 vec2
          vec1 = reduced value from this input vector will be replaced
          vec2 = replace it with vec1[N] where N is index of max/min value from vec2

## Examples

``` LAMMPS
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
compute 2 all reduce min c_press[*] f_ave v_myKE
compute 3 fluid reduce max c_index[1] c_index[2] c_dist replace 1 3 replace 2 3
```

## Description

Define a calculation that \"reduces\" one or more vector inputs into
scalar values, one per listed input. The inputs can be per-atom or local
quantities; they cannot be global quantities. Atom attributes are
per-atom quantities, [computes](compute) and [fixes](fix) may generate
any of the three kinds of quantities, and [atom-style
variables](variable) generate per-atom quantities. See the
[variable](variable) command and its special functions which can perform
the same operations as the compute reduce command on global vectors.

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 *minabs* or *maxabs* options find the minimum or maximum
value across all absolute vector values. The *ave* setting adds the
vector values into a global total, then divides by the number of values
in the vector. The *sumsq* option sums the square of the values in the
vector into a global total. The *avesq* setting does the same as
*sumsq*, then divides the sum of squares by the number of values. The
last two options can be useful for calculating the variance of some
quantity (e.g., variance = sumsq $-$ ave$^2$). The *sumabs* option sums
the absolute values in the vector into a global total. The *aveabs*
setting does the same as *sumabs*, then divides the sum of absolute
values by the number of values.

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) or [fix](fix) or
the evaluation of an atom-style [variable](variable).

Note that for values from a compute or fix, the bracketed index $I$ can
be specified using a wildcard asterisk with the index to effectively
specify multiple values. This takes the form \"\*\" or \"\*n\" or
\"m\*\" or \"m\*n\". If $N$ is the size of the vector (for *mode* =
scalar) or the number of columns in the array (for *mode* = vector),
then an asterisk with no numeric values means all indices from 1 to $N$.
A leading asterisk means all indices from 1 to n (inclusive). A trailing
asterisk means all indices from m to $N$ (inclusive). A middle asterisk
means all indices from m to n (inclusive).

Using a wildcard is the same as if the individual columns of the array
had been listed one by one. For example, the following two compute
reduce commands are equivalent, since the [compute
stress/atom](compute_stress_atom) command creates a per-atom array with
six columns:

``` LAMMPS
compute myPress all stress/atom NULL
compute 2 all reduce min c_myPress[*]
compute 2 all reduce min c_myPress[1] c_myPress[2] c_myPress[3] &
                         c_myPress[4] c_myPress[5] c_myPress[6]
```

------------------------------------------------------------------------

The atom attribute values (*x*, *y*, *z*, *vx*, *vy*, *vz*, *fx*, *fy*,
and *fz*) are self-explanatory. Note that other atom attributes can be
used as inputs to this fix by using the [compute
property/atom](compute_property_atom) command and then specifying an
input value from that compute.

If a value begins with \"c\_\", a compute ID must follow which has been
previously defined in the input script. Computes can generate per-atom
or local quantities. See the individual [compute](compute) page for
details. If no bracketed integer is appended, the vector calculated by
the compute is used. If a bracketed integer 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](Modify). See
the discussion above for how $I$ can be specified with a wildcard
asterisk to effectively specify multiple values.

If a value begins with \"f\_\", a fix ID must follow which has been
previously defined in the input script. Fixes can generate per-atom or
local quantities. See the individual [fix](fix) 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](Modify). See
the discussion above for how $I$ can be specified with a wildcard
asterisk to effectively specify multiple values.

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). 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 the *replace* keyword is used, two indices *vec1* and *vec2* are
specified, where each index ranges from 1 to the number of input values.
The replace keyword can only be used if the *mode* is *min* or *max*. It
works as follows. A min/max is computed as usual on the *vec2* input
vector. The index $N$ of that value within *vec2* is also stored. Then,
instead of performing a min/max on the *vec1* input vector, the stored
index is used to select the $N$th element of the *vec1* vector.

Thus, for example, if you wish to use this compute to find the bond with
maximum stretch, you can do it as follows:

``` LAMMPS
compute 1 all property/local batom1 batom2
compute 2 all bond/local dist
compute 3 all reduce max c_1[1] c_1[2] c_2 replace 1 3 replace 2 3
thermo_style custom step temp c_3[1] c_3[2] c_3[3]
```

The first two input values in the compute reduce command are vectors
with the IDs of the 2 atoms in each bond, using the [compute
property/local](compute_property_local) command. The last input value is
bond distance, using the [compute bond/local](compute_bond_local)
command. Instead of taking the max of the two atom ID vectors, which
does not yield useful information in this context, the *replace*
keywords will extract the atom IDs for the two atoms in the bond of
maximum stretch. These atom IDs and the bond stretch will be printed
with thermodynamic output.

------------------------------------------------------------------------

If a single input is specified this compute produces a global scalar
value. If multiple inputs are specified, this compute produces a global
vector of values, the length of which is equal to the number of inputs
specified.

As discussed below, for the *sum*, *sumabs*, and *sumsq* modes, 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) command with [thermo_modify norm
yes](thermo_modify) set as an option. Or it can be accessed by a
[variable](variable) that divides by the appropriate atom count.

------------------------------------------------------------------------

## Output info

This compute calculates a global scalar if a single input value is
specified or a global vector of length $N$, where $N$ is the number of
inputs, and which can be accessed by indices 1 to $N$. These values can
be used by any command that uses global scalar or vector values from a
compute as input. See the [Howto output](Howto_output) doc page for an
overview of LAMMPS output options.

All the scalar or vector values calculated by this compute are
\"intensive\", except when the *sum*, *sumabs*, or *sumsq* modes are
used on per-atom or local vectors, in which case the calculated values
are \"extensive\".

The scalar or vector values will be in whatever [units](units) the
quantities being reduced are in.

## Restrictions

> none

## Related commands

[compute](compute), [fix](fix), [variable](variable)

## Default

none