# compute improper/local command

## Syntax

``` LAMMPS
compute ID group-ID improper/local value1 value2 ...
```

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

-   improper/local = style name of this compute command

-   one or more values may be appended

-   value = *chi*

        *chi* = tabulate improper angles

## Examples

``` LAMMPS
compute 1 all improper/local chi
```

## Description

Define a computation that calculates properties of individual improper
interactions. The number of datums generated, aggregated across all
processors, equals the number of impropers in the system, modified by
the group parameter as explained below.

The value *chi* is the improper angle, as defined in the doc pages for
the individual improper styles listed on
[improper_style](improper_style) doc page.

The local data stored by this command is generated by looping over all
the atoms owned on a processor and their impropers. An improper will
only be included if all four atoms in the improper are in the specified
compute group.

Note that as atoms migrate from processor to processor, there will be no
consistent ordering of the entries within the local vector or array from
one timestep to the next. The only consistency that is guaranteed is
that the ordering on a particular timestep will be the same for local
vectors or arrays generated by other compute commands. For example,
improper output from the [compute
property/local](compute_property_local) command can be combined with
data from this command and output by the [dump local](dump) command in a
consistent way.

Here is an example of how to do this:

``` LAMMPS
compute 1 all property/local itype iatom1 iatom2 iatom3 iatom4
compute 2 all improper/local chi
dump 1 all local 1000 tmp.dump index c_1[1] c_1[2] c_1[3] c_1[4] c_1[5] c_2[1]
```

## Output info

This compute calculates a local vector or local array depending on the
number of keywords. The length of the vector or number of rows in the
array is the number of impropers. If a single keyword is specified, a
local vector is produced. If two or more keywords are specified, a local
array is produced where the number of columns = the number of keywords.
The vector or array can be accessed by any command that uses local
values from a compute as input. See the [Howto output](Howto_output)
page for an overview of LAMMPS output options.

The output for *chi* will be in degrees.

## Restrictions

> none

## Related commands

[dump local](dump), [compute property/local](compute_property_local)

## Default

none