# compute angle/local command

## Syntax

``` LAMMPS
compute ID group-ID angle/local value1 value2 ... keyword args ...
```

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

-   angle/local = style name of this compute command

-   one or more values may be appended

-   value = *theta* or *eng* or *v_name*

        *theta* = tabulate angles
        *eng* = tabulate angle energies
        *v_name* = equal-style variable with name (see below)

-   zero or more keyword/args pairs may be appended

-   keyword = *set*

        *set* args = theta name
          theta = only currently allowed arg
          name = name of variable to set with theta

## Examples

``` LAMMPS
compute 1 all angle/local theta
compute 1 all angle/local eng theta
compute 1 all angle/local theta v_cos set theta t
```

## Description

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

The value *theta* is the angle for the three atoms in the interaction.

The value *eng* is the interaction energy for the angle.

The value *v_name* can be used together with the *set* keyword to
compute a user-specified function of the angle theta. The *name*
specified for the *v_name* value is the name of an [equal-style
variable](variable) which should evaluate a formula based on a variable
which will store the angle theta. This other variable must be an
[internal-style variable](variable) defined in the input script; its
initial numeric value can be anything. It must be an internal-style
variable, because this command resets its value directly. The *set*
keyword is used to identify the name of this other variable associated
with theta.

Note that the value of theta for each angle which stored in the internal
variable is in radians, not degrees.

As an example, these commands can be added to the bench/in.rhodo script
to compute the cosine and cosine-squared of every angle in the system
and output the statistics in various ways:

``` LAMMPS
variable t internal 0.0
variable cos equal cos(v_t)
variable cossq equal cos(v_t)*cos(v_t)

compute 1 all property/local aatom1 aatom2 aatom3 atype
compute 2 all angle/local eng theta v_cos v_cossq set theta t
dump 1 all local 100 tmp.dump c_1[*] c_2[*]

compute 3 all reduce ave c_2[*]
thermo_style custom step temp press c_3[*]

fix 10 all ave/histo 10 10 100 -1 1 20 c_2[3] mode vector file tmp.histo
```

The [dump local](dump) command will output the potential energy
($\phi$), the angle ($\theta$), $\cos(\theta$), and $\cos^2(\theta)$ for
every angle $\theta$ in the system. The [thermo_style](thermo_style)
command will print the average of those quantities via the [compute
reduce](compute_reduce) command with thermo output. And the [fix
ave/histo](fix_ave_histo) command will histogram the $\cos(\theta)$
values and write them to a file.

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

The local data stored by this command is generated by looping over all
the atoms owned on a processor and their angles. An angle will only be
included if all three atoms in the angle are in the specified compute
group. Any angles that have been broken (see the
[angle_style](angle_style) command) by setting their angle type to 0 are
not included. Angles that have been turned off (see the [fix
shake](fix_shake) or [delete_bonds](delete_bonds) commands) by setting
their angle type negative are written into the file, but their energy
will be 0.0.

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,
angle 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 atype aatom1 aatom2 aatom3
compute 2 all angle/local theta eng
dump 1 all local 1000 tmp.dump index c_1[1] c_1[2] c_1[3] c_1[4] c_2[1] c_2[2]
```

## Output info

This compute calculates a local vector or local array depending on the
number of values. The length of the vector or number of rows in the
array is the number of angles. If a single value is specified, a local
vector is produced. If two or more values are specified, a local array
is produced where the number of columns = the number of values. 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 *theta* will be in degrees. The output for *eng* will be
in energy [units](units).

## Restrictions

> none

## Related commands

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

## Default

none